Compute field offsets once in the layout packer

[tachytelicdetonation]

lay_fields calculated field offsets before lay_pack mutated each offset to account for the constructor tag. This makes lay_pack accept constructor names and field layouts and assign the final offsets while merging word kinds. It removes the intermediate offset calculation and the caller-supplied offsets in arr_lay.

Only bend2/comp.ts changes: +17 / -27 lines (net -10), and 64,941 -> 64,881 ttok. No language features or comments are removed.

Validation against upstream commit 99f9c6cd on macOS arm64:

• Compared CLI compilation of 263 inputs: every direct .bend file under tests/{compile,run,base,show,rfc,cost} plus all 16 runtime benchmarks. All 235 emitted C files and 235 emitted JavaScript files are byte-identical; the other 28 inputs have identical compilation diagnostics and exit status.
• Ran 16 representative layout programs on native C (one CPU thread, GPU off) and JavaScript: all 32 runs match their #| output. Four additional rejection tests match their expected errors and exit status.
• gates/repo.ts: PASS 46 / 46. git diff --check: pass.
• TypeScript 7.0.2 with the repository tsconfig reports the same four diagnostics before and after: two in bend2/bend.ts and missing canvas declarations in two documentation generators. No diagnostics in comp.ts.
• The private-cluster test, performance, and release gates were not run. Benchmark source generation was compared; runtime benchmark timings were not measured.

[VictorTaelin]

Merged into 2.0.25 as one commit under your authorship, thank you. We verified the byte-identical emission over 914 outputs on our side. It landed together with the wide-record fix of #944, so lay_pack now also takes the field lists through lay_wide; the offsets are yours.

Note: this reply was written by an AI after it reported the issue to me and I made the decision. If anything here is wrong, reply and I will review it myself.