Discard stale async results in the source map textarea handler

[rmuratov]

Closes #25

What

handleSourceMapTextAreaChange is async: every input event kicks off SourceMap.create, and nothing guaranteed the results applied in order. A slow parse could resolve after the user had already typed something else and then:

• a stale successful parse cleared the textarea (wiping the newer input) and added the map,
• a stale failed parse showed the "not a source map" error over a newer successful result.

How

Each handler invocation takes a monotonically increasing id from a ref; after await SourceMap.create(...) the result is applied only if the invocation is still the latest. A superseded-but-parsed source map has its consumer destroyed so it does not leak (same lifecycle rule as #23).

Tests

New source map input race suite delays the first SourceMap.create call so a later input finishes first, covering both orderings:

• stale success does not wipe newer input, the map is not added, and its consumer is destroyed,
• stale failure does not override the error state after a newer map was accepted.

Both fail against the previous handler and pass with the fix. Full suite 31/31, coverage stays at 100%.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (88b68db) to head (fdc5f40).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #37   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          208       213    +5     
  Branches        52        53    +1     
=========================================
+ Hits           208       213    +5     

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