Enable tsconfig strict mode + reduce usage

[loadfix]

Context

xlsxjs has ~58 `any` hits across 21 source files — concentrated in `workbook-parser.ts` (22), `html-renderer.ts` (10), `number-format.ts` (8).

All three JS renderers run `tsconfig.json` with `strict: (unset)` and `noImplicitAny: false`. Across the trio, pptxjs is the clean outlier (~3 `any`s); docxjs and xlsxjs both have meaningful surface area.

Expected approach

1. Ratchet file: commit `.any-budget` listing current counts per file. CI guards against regression. Contributors may NOT introduce new `any` without reducing another.
2. Gradual typing: review the 58 sites. Cluster: formula-eval (`AST` handling), workbook-parser (untyped XML traversal), html-renderer (DOM manipulation). Replace easy ones with `unknown` + refinement, or explicit type definitions.
3. Flip noImplicitAny: true once count drops below (say) 20.
4. Flip strict: true once count hits 0.

See also

• Same issue open on docxjs
• pptxjs is the target standard (strict=true feasible today)

Surfaced by: 2026-05-05 consistency audit (item 10).