[bytecode]: section 5 function debug name stub (PHX-070-p7)

[JamalLyons]

Summary

• Extends PHX0 section 5 to sub-version 2 with a per-function_id debug name table appended after the PC span map (v1 payloads still decode).
• Codegen records function display names in dev builds; linker merge_from merges names; verifier rejects unknown function ids in the name table.
• Unit/integration tests cover round-trip, v1 backward compat, merge, codegen emission, and link merge.

Test plan

• cargo test -p phx-bytecode
• cargo test -p phx-compiler --test codegen codegen_emits_function
• cargo test -p phx-compiler --test link link_dev_profile
• just pre-commit

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