docs: add forms and validation conventions guide

[playmaker410]

Closes #329

Summary

Adds the forms and validation guide requested in issue #329. Documents the shared validation contract so new forms stop inlining their own checks.

What's changed

docs/forms.md — new guide

• Section 1 — forms-to-validator mapping table (Create Service, Edit Service, Usage Metering, Webhooks)
• Section 2 — full signatures and invariants for validateIdentifier, parseNonNegativeInt, parsePositiveInt, including Number() coercion edge cases (hex, binary, leading-plus, trailing .0)
• Section 3 — error-message conventions: sentence case, trailing period, and the documented inconsistency in parsePositiveInt's default message
• Section 4 — TextField ARIA wiring (aria-invalid, aria-describedby, role="alert")
• Section 5 — decision table: shared helper vs. inline validation
• Section 6 — end-to-end worked example: state wiring → TextField → submit guard → unit test

docs/components.md

• Added cross-reference from the TextField entry to docs/forms.md

src/lib/validateNumber.ts

• Fixed incorrect JSDoc: "1e2" was listed in the Rejected examples but correctly evaluates to the integer 100 and is accepted

src/lib/__tests__/validateNumber.test.ts

• Added 5 new edge-case tests: ParseResult type assertion, hex/binary coercion (0xff, 0b101), leading-plus (+1, +0), and parsePositiveInt("00000") → rejected
• Total: 29 tests, all passing

jest.config.ts

• Locked validateNumber.ts and validateId.ts at ≥ 95% coverage thresholds (statements / branches / lines) and 100% functions

Test output

PASS src/lib/__tests__/validateNumber.test.ts (22 tests)
PASS src/lib/__tests__/validateId.test.ts (7 tests)

validateNumber.ts  100% stmts | 100% branch | 100% funcs | 100% lines
validateId.ts      100% stmts | 100% branch | 100% funcs | 100% lines

npm run lint, npm test, and npm run build all pass. The 3 pre-existing test failures in docs/page.test.tsx, export/page.test.tsx, and events/page.test.tsx are unrelated to this PR and exist on main before this change.