perf(gfql): validate a query's ops once per call, not twice - #2088

Every AST object is validated TWICE inside a single execution. `gfql` builds a Chain
from the caller's ops, which validates them; the execution path then constructs a
throwaway Chain purely to validate them again, and deliberately unwraps an
already-validated Chain to do it. Measured redundancy factor 1.7-1.8x by counting
ASTSerializable.validate calls in one execution.

This is not a benchmark artifact, and that was checked before building. The SNB harness
reuses AST objects across repetitions, so a memo keyed on object identity would show a
large benchmark win and do nothing for a real caller; that was rejected. What is removed
here is redundant WITHIN a single call, so a caller that passes a plain list and reuses
nothing pays both passes today.

The re-validation exists to catch a caller that built a Chain, mutated its ops, and then
executed it. That caller is unaffected and still raises, pinned for both entry points.
The signal is explicit and narrow: Chain.gfql_validated(), set only by gfql on a Chain it
just built, never inferred from the Chain itself, and it cannot be forged onto a Chain
constructed with validate=False. Two sites needed it -- the polars branch, and the pandas
path, which unwraps the Chain before validating and so loses any marker.

Measured in-process, arms alternated, 6 rounds x 31 reps, each against its own A/A
control (LDBC SNB SF0.1, polars):

  seed-lookup      0.995 -> 0.897 ms  (-0.098, A/A floor 0.034)
  message-replies  1.714 -> 1.640 ms  (-0.074, A/A floor 0.011)
  recent-replies   3.488 -> 3.390 ms  (-0.098, A/A floor 0.022)

Three to seven times its own floor on each. It is a fixed per-query cost, so it helps
every query and every engine.

No end-to-end claim: a cross-process A/B against master cannot resolve 0.1 ms, its own
A/A floor running +3.3% to +12.5%.

Tests: 8 new, covering both sides -- gfql validates once, chain called directly still
re-validates, a Chain mutated after construction is still caught through both entry
points, the mark cannot be forged, and malformed queries raise identical error codes.
Validation suites 746 pass. Broad compute suite 14980 pass / 32 fail, an identical
failure set to untouched master on this box.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_012Me1E7ZdDuGqJGu3mMEzhp

…ehaviour boundary

Review feedback, both items.

DYNAMIC ACCESS. The skip was decided by `getattr(ops, "_gfql_validated_in_call", False)`
-- an attribute probe used as control flow, which the review skill rejects: control-flow
checks must use structured signals. `chain` accepts a list or a Chain and only the latter
can carry the mark, so the check is now a typed predicate on the known type,
`Chain.ops_were_validated_in_this_call(ops)`, doing an isinstance narrow and reading a
declared bool attribute. Both call sites use it; no probe remains.

TESTS ASSERTED IMPLEMENTATION, NOT BEHAVIOUR. The old pins counted Chain constructions.
The real boundary is "could these ops have changed since they were validated?", and the
suite now tests what a CALLER observes on each side of it:

  could have changed  -> a Chain mutated after construction still raises, through BOTH
                         entry points; a Chain reused across calls and mutated between
                         them still raises; a plain list reused and mutated still raises;
                         three malformed shapes raise the same code through both entries
  could not have      -> well-formed ops return the same rows through both entry points,
                         and gfql and chain agree on the same ops

One implementation assertion remains, labelled as such: the engagement pin that fails if
the optimization is removed entirely. Without it the behaviour tests would all pass with
the change reverted.

Verified in both directions rather than assumed. Over-applying the skip (skipping for
every caller) fails `test_a_chain_mutated_after_construction_still_raises[chain]`.
Removing the optimization fails the engagement pin. 15 pass as shipped.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_012Me1E7ZdDuGqJGu3mMEzhp

Comments-only. The predicate `ops_were_validated_in_this_call` already says what the
check is, and the tests already say what the contract is, so the paragraphs explaining
both were doing no work.

Removed: the two-line attribute comment (the attribute names say it), the five-line
`gfql_validated` docstring (now one line), the four-line predicate docstring (the name is
the documentation), and the appended halves of the two call-site comments -- those are
back to the one-line originals they had before this branch touched them.

Test file: module docstring cut from twelve lines to three, six per-test docstrings that
restated their own test name deleted, two trimmed, three section banners removed.

Behaviour unchanged: chain.py is 3 insertions / 21 deletions, all prose. Contract and
reuse suites 43 pass; lint, guards and mypy clean.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_012Me1E7ZdDuGqJGu3mMEzhp