Allow unstable-fn to target primitives

[saulshanabrook]

Context

unstable-fn function containers could target table functions, but primitive overloads needed matching typechecking and runtime resolution support. General follow up on #832

Changes

• Extends unstable-fn type constraints to account for primitive overloads.
• Resolves primitive function containers for scheduled rules and merge functions.
• Keeps resolver APIs private to this implementation.
• Adds tests for primitive-backed function containers and merge-function use.

Validation

• cargo test --lib test_unstable_function_container_primitives
• cargo test --test files --features bin unstable_fn
• cargo fmt
• git diff --check

[codspeed-hq]

Merging this PR will improve performance by 8.47%

⚡ 1 improved benchmark
✅ 24 untouched benchmarks
⏩ 215 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	tests[repro-665-set-union]	627.6 ms	578.6 ms	+8.47%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing saulshanabrook:codex/split-unstable-fn-primitive-targets (671b2b5) with main (8c1c70b)²}

Footnotes

1. 215 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (d330f2c) during the generation of this report, so 8c1c70b was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩