fix(api): point /api/cache/stats at the real semantic cache

[Poid-ZA]

Summary

• /api/cache/stats read cacheLayer.getPromptCache(), a singleton with zero callers anywhere in the request pipeline — it always reported 0 hits/misses regardless of real cache activity.
• The dashboard /dashboard/health and /dashboard/usage (BudgetTelemetryCards) both fetch this endpoint directly, so they were always displaying a dead cache instance instead of the real one.
• The cache actually wired into chatCore.ts on every request is semanticCache.ts. Added getMemoryLayerStats() there, matching the exact field shape (size/maxSize/bytes/maxBytes/hits/misses/evictions/hitRate as a number) both existing consumers already depend on, and repointed the route + its DELETE handler at it.
• Removed the now-unused getPromptCache() singleton from cacheLayer.ts. The LRUCache class it wrapped is unaffected and still used directly by semanticCache.ts.

Test plan

• node --import tsx/esm --test tests/unit/semantic-cache.test.ts — new tests for getMemoryLayerStats() written first (failed with the missing export), then implemented and passing (23/23 total in file)
• npm run typecheck:core — clean
• npx eslint on all four changed files — clean
• Confirmed via grep that nothing else in src/, open-sse/, or tests/ referenced cacheLayer.getPromptCache before removing it

[diegosouzapw]

Thanks for this contribution — the change itself is wanted! I'm closing this PR for a mechanical reason only (nothing wrong with your fix): the branch was cut from a very old point of the repo, so its tree now conflicts with release/v3.8.50 on dozens of unrelated files (docs, CI workflows, baselines, lockfiles). Merging it as-is could silently roll back months of unrelated work, and resolving 70+ conflicts by hand is riskier than recreating a branch that only contains your change.

How to recreate it cleanly (5 minutes):

# 1. Sync your fork's view of the upstream release branch
git fetch upstream release/v3.8.50        # add the remote first if needed:
                                          # git remote add upstream https://github.com/diegosouzapw/OmniRoute.git

# 2. Cut a FRESH branch from the current release tip (never from main or an old branch)
git switch -c <your-branch-name-v2> upstream/release/v3.8.50

# 3. Bring over ONLY your change. Two easy options:
#    a) cherry-pick your original commit(s):
git cherry-pick <sha-of-your-commit>      # repeat per commit; resolve only YOUR files
#    b) or re-apply the diff of just your files:
git checkout <old-branch> -- <path/to/your/file1> <path/to/your/file2>

# 4. Sanity: the diff must contain ONLY your files
git diff upstream/release/v3.8.50 --stat

# 5. Run the focused checks, push, and open the PR against release/v3.8.50
npm run typecheck:core && node --import tsx/esm --test tests/unit/<your-test>.test.ts
git push -u origin <your-branch-name-v2>

Two extra notes that will save you time:

• Several base-side CI problems were fixed today (leaked failing tests, a wrong ESLint baseline, a missing MDX frontmatter). Any red you saw on this PR before was inherited from the base, not caused by your change — a fresh branch from today's tip starts green.
• Please tick "Allow edits by maintainers" on the new PR: it lets us refresh your branch when the base moves, instead of asking you every time.

Reopen at will — or open the recreated PR and mention this one so the history links up. Happy to help if any step misbehaves.