perf: metadata caching & hot path optimisations

[matthew2564]

Description

• Series of performance optimisations. Benchmarked develop and then re-run on branch. All 700+ tests passing.

So the flow was:

1. Create benchmark.ts on this branch
2. git stash
3. Run benchmark → baseline numbers
4. git stash pop
5. Run benchmark

Scenario	Before (upstream/develop)	After (this branch)	Speedup
Valid object (async, 13 props + nested)	113,251 ops/s	337,382 ops/s	2.98x
Invalid object (async, 3 errors)	102,439 ops/s	254,488 ops/s	2.48x
Valid with strictGroups (async)	114,397 ops/s	349,113 ops/s	3.05x
Valid object (sync, 13 props + nested)	120,335 ops/s	369,863 ops/s	3.07x
Invalid object (sync, 3 errors)	105,770 ops/s	262,942 ops/s	2.49x
Valid with strictGroups (sync)	119,076 ops/s	359,657 ops/s	3.02x

Checklist

• the pull request title describes what this PR does (not a vague title like Update index.md)
• the pull request targets the default branch of the repository (develop)
• the code follows the established code style of the repository
  • npm run prettier:check passes
  • npm run lint:check passes
• tests are added for the changes I made (if any source code was modified)
• [] documentation added or updated
• I have run the project locally and verified that there are no errors

[matthew2564]

@braaar I've hopefully now fixed the issues detected in the pipeline (formatting & missing test cov) 👍

[braaar]

Could you fix the merge conflicts?

[braaar]

Could you go over the tests to increase the coverage a bit?