Define string runtime semantics #29

Goal

Define foundation-level str runtime semantics.

Settled semantics

• str is an owned immutable string value.
• &str is borrowed view used when passing or inspecting existing text.
• "..." produces str; c"..." produces C string value.
• b'x' produces byte; 'x' produces character value.
• len(&value) reports UTF-8 byte length.
• as_bytes(&value) returns zero-copy borrowed byte view.
• as_chars(&value) returns owned decoded character array.
• String ranges return borrowed &str views with ordered byte bounds and UTF-8 boundary checks.
• Direct string indexing and borrowed-string comparison remain rejected until separately specified.

Delivered

• Owned str checking, literals, printing, carrier ABI, allocator-aware cleanup, and direct raw carrier addresses.
• Canonical len, as_bytes, and as_chars free functions with pipe adaptation.
• Borrowed string ranges, UTF-8 boundary traps, and temporary-borrow escape diagnostics.
• Positive, negative, runtime, 32-bit, and 64-bit regression coverage.
• PRs Fix string ABI and place traversal duplication #68, Fix 32-bit ABI and intrinsic conformance #69, Complete end-to-end compiler review remediation #71, Fix cloning of open-ended default ranges #72, and Add free builtins and pipe call support #77 contain delivered work.

Follow-ups

• Add runtime string construction and concatenation #81 adds runtime string construction and concatenation.
• Define explicit string C and FFI bridge #82 defines explicit C and FFI bridge rules.

Foundation contract is complete. Future work stays in linked follow-up issues.