docs(changeset): correct two claims the rescued #17818 changeset could not support

The changeset was committed unreviewed out of a worktree whose round died. Its
before/after table is accurate — all eight rows reproduce — but two sentences
around it did not survive review:

* "against the built artifact" named a provenance nobody measured. The reading
  on record evaluates each fold at this change's implementation and again at
  its merge base, against the TypeScript sources the build and the test run
  both consume. The sentence now names that.

* "the level follows the widening rule" had the direction backwards. This diff
  NARROWS: an off-vocabulary key that previously resolved an inherited member
  now gets each function's own declared refusal value, and nothing that
  answered before answers differently. `minor` is right, but it is carried by
  the change's declared contract-review status, not by a widening.

No change to the table, to the guard, or to any pin.

Claude-Session: https://claude.ai/code/session_01KB5PFtxuy1x3dcR5gxudx6
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

.changeset/17818-prototype-fallthrough-lookups.md

@@ -12,7 +12,9 @@ not in the vocabulary resolved a member of `Object.prototype` instead of
 falling through — and the `?? fallback` each function already writes never
 fired, because the inherited member is truthy.
 
-Measured on Node v22.22.2 against the built artifact, before and after:
+Measured on Node v22.22.2, before and after — each fold evaluated at this
+change's implementation and again at its merge base, against the TypeScript
+sources that the build and the test run both consume:
 
 | call | before | after |
 |:--|:--|:--|
@@ -31,13 +33,15 @@ fallback was invented. `resolveDiscoveryEnvironment` is the sharpest case: its
 own docblock promises "a value guaranteed to satisfy
 `DiscoveryEnvironmentSchema`", and for `constructor` it returned a `Function`.
 
-⚠️ **Why `minor` and not `patch`.** These are published exports whose observable
-answer changes for a class of input, so the level follows the widening rule for
-a declared contract change rather than the bug-fix default. Nothing in the
-declared vocabulary moves: every canonical operator, every `EnvironmentType`
-bucket, both operator shorthands and every manifest collection spelling answers
-byte-identically to before, and the only inputs whose answer changes are the
-four prototype-member spellings above, which no signature ever admitted.
+⚠️ **Why `minor` and not `patch`.** The level is carried by this change's
+declared contract-review status, ⛔ not by a widening — the guard only NARROWS.
+An off-vocabulary key that previously resolved an inherited member now gets each
+function's own declared refusal value, and nothing that answered before answers
+differently. Nothing in the declared vocabulary moves: every canonical operator,
+every `EnvironmentType` bucket, both operator shorthands and every manifest
+collection spelling answers byte-identically to before, and the only inputs
+whose answer changes are the four prototype-member spellings above, which no
+signature ever admitted.
 
 The guard is the `Object.prototype.hasOwnProperty.call(table, key) && table[key]`
 shape already landed in `src/data/type-compat.ts`, and carries that site's two