fix(internal): avoid RangeError when diffing large arrays

[r3wretrhy]

Myers diff() allocates a Uint32Array of length (M * N + …) * 2. For two arrays of length 2^18 that is hundreds of gigabytes, so assertEquals throws RangeError (or panics Deno) instead of AssertionError:

assertEquals(
  Array.from({ length: 2 ** 18 }, () => Math.random()),
  Array.from({ length: 2 ** 18 }),
);
// RangeError: Array buffer allocation failed
//     at new Uint32Array
//     at diff

Even after that allocation is avoided, buildMessage() did messages.push(...diffMessages), which overflows V8's argument limit once the formatted diff has tens of thousands of lines (RangeError: Maximum call stack size exceeded).

Fixes #5942

Fix

• Cap the Myers route buffer at 2^26 Uint32s (256 MiB). If the table would be larger, or new Uint32Array throws RangeError, fall back to a linear prefix/suffix hunk. A single inner change still renders as one hunk.
• Build the assertion message with concat instead of spreading the line array.

Small diffs are unchanged: they still go through Myers.

Tests

• deno test -A --parallel --doc internal/ assert/ — 241 passed
• deno fmt --check on the changed files
• deno lint — 1169 files
• deno task lint:circular, lint:mod-exports, lint:export-names, lint:unstable-deps

New tests fail on main (RangeError) and pass here:

• diff() does not throw RangeError for large disjoint arrays (n = 2 ** 18)
• diff() keeps a single inner change as one hunk when the Myers table would overflow
• buildMessage() … large diff does not throw RangeError
• assertEquals() throws AssertionError for large unequal arrays

AI usage: assisted analysis/tests with a coding assistant.

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[codecov]

Codecov Report

❌ Patch coverage is 78.00000% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.02%. Comparing base (ca58f94) to head (bbc94bf).

Files with missing lines	Patch %	Lines
internal/diff.ts	76.08%	6 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7300      +/-   ##
==========================================
- Coverage   95.03%   95.02%   -0.02%     
==========================================
  Files         617      618       +1     
  Lines       51637    51687      +50     
  Branches     9359     9368       +9     
==========================================
+ Hits        49075    49113      +38     
- Misses       2021     2027       +6     
- Partials      541      547       +6     

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