fix(codec): pad zero integers with zeros not sign extension

[d1r1]

• zero uint16..uint64 and int16..int64 stop encoding as 0xff padding
• indexed event topics of zero values now match solidity
• fixes FLU-1111
• pins the rule against alloy-sol-types, not a self round-trip

Summary by CodeRabbit

• Bug Fixes
  • Corrected integer encoding so negative values use sign extension, while zero and unsigned values use zero padding.
  • Fixed canonical zero encoding across supported integer types, alignments, byte orders, and widths.
  • Ensured zero-valued members remain correctly encoded in arrays and tuples, including dynamic and fixed-size containers.

[coderabbitai]

📝 Walkthrough

Walkthrough

Changes

Integer Padding Correction

Layer / File(s)	Summary
Correct integer padding and primitive coverage crates/codec/src/primitive.rs	Integer encoding uses 0xFF only for negative values and 0x00 for zero or unsigned values. Tests cover widths, byte orders, alignments, boundaries, and canonical zero encoding.
Topic and container regression coverage crates/codec/tests/topic.rs	Topic tests cover zero padding, negative sign extension, positive boundaries, and zero-valued members in arrays and tuples.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: dmitry123

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main codec fix: zero integers use zero padding instead of sign extension.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/int-zero-encoding

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[d1r1]

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[github-actions]

Criterion results (vs baseline)

running 131 tests

Heads-up: runner perf is noisy; treat deltas as a smoke check.