Add stage-by-stage logging to the search text-layer pipeline (#147 diagnostics)

[philips-clanker]

Summary

Diagnostic-only change for #147 (crash/"boot loop" opening a specific .note file on iOS). Not a fix - we don't have a confirmed root cause yet, and this is intended to get one.

What we know so far: the reporter's note is only 8 pages; the render-completion log (main.ts:1027, after parsing/rasterization/ImageBitmap decode/canvas draw for all pages succeed) is the last thing that ever printed - even with Safari's Web Inspector attached live to the device. That rules out parsing/rasterization/canvas rendering as the cause for this report, and points at whatever runs immediately after: the fire-and-forget pipeline that builds a text-only PDF (assembleTextOnlyPdf/pdf-lib) purely so pdf.js can provide a search/select text layer, plus each visible page's own text-layer build in ensureTextLayer().

The concerning part: ensureTextLayer() already wraps its work in try/catch and logs Failed to build text layer for page N: ... on any ordinary thrown exception - and that log never appeared either, with the inspector attached. Whatever's happening is severe enough to leave no catchable JS error and no further console output at all - consistent with a hard native/WASM-level crash or an unresponsive-script watchdog kill, not a normal bug we could just try/catch our way out of blind.

Changes

Adds console.debug checkpoints stage-by-stage:

• assembleTextOnlyPdf: per-page, plus before saving
• The pdfDocPromise IIFE in onLoadFile(): before/after building the PDF, loading pdf.js, and parsing it
• ensureTextLayer(): before awaiting the PDF and before/after building each page's own text layer

Whichever line prints last next time narrows this from "somewhere in a ~6-stage pipeline with zero visibility" down to one specific stage, or one specific page's recognition data - actionable next-step info instead of another log line identical to what we already have.

Test plan

• npx tsc --noEmit - clean
• npx eslint src/main.ts - 0 errors
• npx vitest run - 95 passed
• npm run build - clean
• Reporter to reproduce with this build and share whichever console.debug line prints last