Memoize stack trace parsing and transformation

[rmuratov]

Closes #30

Problem

StackTrace.create and transform (which walks every frame and calls originalPositionFor on the consumer) ran on every render of App — including renders caused by unrelated state, e.g. every keystroke in the source map textarea or a theme toggle. For large stack traces with several source maps this causes noticeable input lag.

Fix

Wrapped both steps in useMemo:

• parsing is keyed on stackTraceInputValue,
• transformation is keyed on sourceMaps and the parsed stackTrace.

Debouncing from the issue was left out: with memoization the expensive work only runs when its actual inputs change, and re-parsing on each stack trace keystroke is the desired behavior (the result pane updates live).

Tests

Two integration tests (memoization suite): after entering a stack trace and uploading source maps, an unrelated re-render (typing into the source map textarea) must not call StackTrace.create or the consumer's originalPositionFor again. originalPositionFor is spied on via the consumer prototype, same technique as the existing destroy spy. Both tests fail on main without the fix (verified via git stash).

Correctness of the memo dependencies is already covered by existing tests: the result updates after editing the stack trace, uploading, and deleting source maps.

Coverage stays at 100%.

[cloudflare-workers-and-pages]

Deploying sourcemap-tools with    Cloudflare Pages

Latest commit:	937597c
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Preview URL:	https://a61b9d9b.sourcemap-tools.pages.dev
Branch Preview URL:	https://memoize-stack-trace-transfor.sourcemap-tools.pages.dev

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8606456) to head (937597c).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #38   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files            9         9           
  Lines          213       213           
  Branches        53        53           
=========================================
  Hits           213       213           

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