[native] Remove the libc++ dependencies from the CoreCLR host's startup path - #12570

The inline-buffer-plus-heap-fallback `path_buffer` was more machinery than these
three values need. They are assigned exactly once, early during startup, and only
read afterwards, so the inline buffer only ever saved a single `malloc` per value
while costing 3 KB of `.bss`.

Replace it with plain `const char*` members initialized to `""`. Pointers to a
string literal are constant-initialized just like the aggregate was, so the guard
variables and `atexit` registrations stay gone, which was the whole point of the
change. The values are duplicated with a new `Util::duplicate_string()` helper,
which aborts if the allocation fails.

Also format the APK library directory with `snprintf` instead of open-coded
`memcpy` calls - the exact length is computed up front, so the buffer is already
known to be the right size.

Undefined libc++ references are unchanged at 31.

Co-authored-by: Copilot <[email protected]>
Copilot-Session: 0a35a0db-502d-48c0-8468-e73b5dd0ab2e

Addresses review feedback:

- `app_lib_directories_size * sizeof (const char*)` is now computed with
  `Helpers::multiply_with_overflow_check`.
- A zero-length array is handled explicitly. `malloc (0)` may legitimately return
  `nullptr`, which the previous code would have misreported as an allocation
  failure; `setup_apk_directories ()` already aborts with a more accurate message
  when no directory ends up being added.

Co-authored-by: Copilot <[email protected]>
Copilot-Session: 0a35a0db-502d-48c0-8468-e73b5dd0ab2e

The view returned by `get_string_view ()` pointed at the UTF characters owned
by the wrapper, so it dangled as soon as the wrapper released them. Nothing
relied on the view being a view: two of the three callers immediately passed it
to a path helper, and the third only needed a suffix comparison. Return the C
string instead and let the callers build a view when they need one.

`setup_apk_directories ()` used `std::string_view::ends_with ()`, so add a
`Util::ends_with ()` that works on plain C strings.

Co-authored-by: Copilot <[email protected]>
Copilot-Session: 0a35a0db-502d-48c0-8468-e73b5dd0ab2e

The hand-written copy existed to support a caller that passed a pointer and a
length rather than a C string, but that caller formats its buffer with
`snprintf ()` and only reaches the call when the result fits, so the buffer is
already NUL terminated. With every caller passing a C string there is nothing
left for `std::string_view` to do and the copy is just `strdup ()`.

Keep the wrapper rather than calling `strdup ()` directly: it aborts on
allocation failure, which saves each of the four callers from checking for
null.

Co-authored-by: Copilot <[email protected]>
Copilot-Session: 0a35a0db-502d-48c0-8468-e73b5dd0ab2e

The only caller of `get_full_dso_path ()` iterates over a container of
`const char*` directories and wrapped each one in a `std::string_view` purely
to satisfy the signature. Take a C string instead and measure it once inside
`format_full_dso_path ()`.

`dso_path` stays a view: it originates in the DSO cache lookup, which compares
name mutations built with `substr ()`, so a view is the right type there.

Co-authored-by: Copilot <[email protected]>
Copilot-Session: 0a35a0db-502d-48c0-8468-e73b5dd0ab2e

Co-authored-by: Copilot App <[email protected]>

Co-authored-by: Copilot App <[email protected]>

Co-authored-by: Copilot App <[email protected]>

Part of #12533. This clears `host.cc` entirely, taking the CoreCLR host
from 21 undefined libc++ references to 12 - all of which now live in
`assembly-store.cc`.

Three separate uses, two of which only compile in Release:

* `scan_filesystem_for_assemblies_and_libraries` built the assembly store
  path with a `std::string`. It now uses `Util::join_paths`, added earlier
  in this stack, which keeps the path on the stack unless it does not fit
  and frees only when the returned pointer differs from the stack buffer.

* `APP_CONTEXT_BASE_DIRECTORY` was held in a function-local
  `static std::string`. The static needs process lifetime because the value
  has to outlive `coreclr_initialize`, and a `std::string` has a non-trivial
  constructor, so it forced a guard variable - this was the only source of
  `__cxa_guard_acquire`/`__cxa_guard_release` in the file. A plain `char*`
  is constant-initialized, so the guard pair disappears.

* The FastDev block used a `std::string` plus two `std::vector<const char*>`.
  It is inside `if constexpr (Constants::is_debug_build)`, so it contributes
  nothing to Release, but it has to go before libc++ can be dropped from
  Debug builds too. The property arrays are a fixed `prop_count + 1` entries,
  so they become `calloc`ed arrays.

`FastDevAssemblies::build_tpa_list` consequently returns a `malloc`ed string
the caller owns rather than filling a `std::string&` out-parameter. The TPA
list has no useful upper bound - one absolute path per assembly in the
override directory - so it is accumulated through a small growable buffer
that doubles on demand. Allocation failure there falls back to the
probe-only path rather than aborting, since FastDev is a debug convenience.

`open_assembly` swaps `new uint8_t[]`/`delete[]` for `malloc`/`free`. That
buffer is handed to CoreCLR and never freed (see the existing TODO), so this
is not a behavioural change. It also gains a null check, which fixes a file
descriptor leak on the allocation-failure path that `new` would have turned
into a `std::bad_alloc` abort.

`<string>` and `<vector>` are no longer included by `host.cc`, and `<string>`
is gone from `fastdev-assemblies.hh`, which `host.cc` was picking it up from.

Undefined libc++ references in the CoreCLR host, Release:

| object | before | after |
|---|---:|---:|
| `host.cc.o` | 9 | **0** |
| `assembly-store.cc.o` | 12 | 12 |
| **total** | **21** | **12** |

`libnet-android.release.so` goes from 523,224 to 520,112 bytes (-3,112).

Debug is not built locally, so `host.cc` and `fastdev-assemblies.cc` were
compiled standalone with `-DDEBUG -DDEBUG_BUILD` using the flags from
`compile_commands.json`. Both compile clean and report 0 undefined libc++
references.

* CoreCLR, MonoVM and NativeAOT all build clean.
* Debug-mode compilation of both affected files verified as described above,
  which matters because `fastdev-assemblies.cc` is only added to the build
  under `if(DEBUG_BUILD)` and is therefore never compiled in Release.

Co-authored-by: Copilot <[email protected]>
Copilot-Session: 0a35a0db-502d-48c0-8468-e73b5dd0ab2e

Co-authored-by: Copilot App <[email protected]>

Co-authored-by: Copilot App <[email protected]>

Reset FastDev TPA state on every rebuild and keep runtime property allocation failures on the probe-only path.

Co-authored-by: Copilot App <[email protected]>

`FastDevAssemblies::tpa_in_use` only exists under `#if defined(DEBUG)`, but
host.cc referenced it from an `if constexpr (Constants::is_debug_build)` branch.
The discarded branch of an `if constexpr` in a non-template function is still
name-looked-up, so Release builds failed with "no member named 'tpa_in_use'".

Route the reset through a new `discard_tpa_list ()` which, like `open_assembly ()`
and `build_tpa_list ()`, has a no-op Release stub.

Co-authored-by: Copilot App <[email protected]>

Preserve empty-file handling without relying on malloc(0), close the override file descriptor after reading, and document the process-global lifetime assumed by the CoreCLR application base directory.

Co-authored-by: Copilot App <[email protected]>

Force the existing path helper onto its heap-owned path so separator handling stays centralized while the CoreCLR property retains explicit process-lifetime ownership.

Co-authored-by: Copilot App <[email protected]>