table: keep dense lookup for wide messages

[fallintoplace]

What

Keep dense lookup for low field numbers in messages with 255 or more fields.

Why

Wide messages currently fall back to binary search for every field.

Implementation

Keep indices for fields below 64 in the dense lookup table.

Benchmark

Not applicable.

[github-actions]

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[iainmcgin]

[claude code] Thanks for this. The change is correct: the dense array is filled only for field numbers below 64, entries are sorted by field number, so such a field is at index 62 or lower and its index-plus-one always fits a u8, whatever the number of entries. The removed assert was redundant with the const checks that remain.

It is on hold for ordering, not for content. It conflicts with the open table codec stack (#475 to #478) in Table::new and in two test files, and that stack merges first. Once it is in I will rebase this PR and push one maintainer commit, then approve it:

• reword the two comments that still describe the 255-entry limit (buffa-test/build.rs, and the W254/W255/W256 tests, which then stop straddling a boundary);
• u8::try_from in place of the as u8 cast in the codegen-side mirror;
• drop the Changed fragment: the table codec is unreleased, so the stack's Added fragment covers it.

There is no action for you in the meantime.

[iainmcgin]

[claude code] Thanks for this, the lookup change itself is unchanged. I pushed a merge of main and one maintainer commit on top:

• Merge with main: the oneof support that landed since added a check next to the assert this PR removes in Table::new. The resolution keeps that check and removes only the 255-entry assert, which the two remaining dense checks already cover.
• dense_lookup in the generator converts the index with u8::try_from and returns a CodeGenError when it does not fit, which takes a descriptor with repeated field numbers. A test builds such a descriptor.
• The W254, W255 and W256 tests in buffa-test straddled the old 255 rule. They now also set fields 63 and 64, the two sides of the end of the dense array, and their names and the comments in build.rs say what they cover after this PR. Two Table::new tests cover a dense array beside more than 255 entries.
• The changelog fragment is removed: the table codec has not been released, so a change to it is not a changelog entry.