fix(platform): bound template whitespace recovery

[yannickmonney]

Malformed template spans could repeatedly scan the same long whitespace suffix during lexical recovery. When a later delimiter existed, candidate validation could also repeatedly trim that suffix. A reader scoped to each template scan now caches the whitespace interval and supplies the exact trimmed source boundary to the existing parser.

Legacy-first segmentation, complete comments, source/depth/token limits, exact offsets and valid long padding remain covered. This bounds the repeated shared-suffix work; it does not claim a universal latency bound for every possible input.

Validation:

• Normal Vitest server project: 69 tests passed across tokens, parse and limits, including the existing compatibility fuzz and real-runner boundary cases.
• Exact-base negative control failed all five new regressions. A cache-only control retaining the old candidate trim failed three, proving that both recovery paths need the fix.
• Bounded 16/64 failed-span probes with 16 KiB padding fell from 131,824–540,800 whitespace checks to 16,385 in all six samples.
• Scoped lint, format, manual reference checks and normal commit hooks passed. Independent source review accepted the frozen patch; complete hosted checks remain required before merge.

[yannickmonney]

This reviewed source is now included in #4565 at 7a526754c1c453e547e6ebb0234bf38c730009e3, together with the other release safety repairs. Independent composition review verified the source blobs are unchanged and the shared coverage register retains every addition.

I am marking this PR as draft to prevent a separate merge and cancelling only its superseded CI runs. The replacement must pass its own seven complete workflows; this PR's checks are retained as historical evidence. This change has not shipped. This PR will be closed as superseded after #4565 merges.

[yannickmonney]

Implemented in #4565, merged as 0ee5fd9656dc12f08976155d956ee76fe39f4ea3, including the reviewed test-instrumentation correction and the parser coverage row. Closing this superseded draft.