feature - panic strategy and allocator hooks for freestanding targets

[dannymeijer]

Area

• Compiler (frontend/backend/codegen)
• Runtime / Core crates (stdlib/core/derive)
• Tooling (CLI/formatter/test runner)

Problem statement

Freestanding targets cannot assume the ordinary hosted panic path or a default allocator. A restricted artifact may need panic = abort, a custom panic halt/report hook, no allocator, a provided allocator, or an allocator initialized by target-specific code. If this remains implicit, no-std builds and kernel-facing work will fail late or depend on backend accidents.

#686 needs minimal panic and allocator handling for a smoke artifact, but 0.8 also needs a dedicated hook model so target profiles, package metadata, runtime layers, and diagnostics agree on what a program requires.

Proposed solution

Add experimental compiler/runtime hooks for panic strategy and allocator requirements under freestanding target profiles.

The first slice should define and plumb:

• panic strategy metadata, such as hosted unwind/report, abort, halt, or target-provided handler;
• allocator requirement metadata, such as none, compiler/runtime-provided, package-provided, or target-provided;
• diagnostics when code requires allocation or panic behavior unavailable under the selected target profile;
• build report fields for panic and allocator requirements;
• a minimal target-provided panic/allocator hook path where required by feature - no-std freestanding build mode and restricted artifact smoke test #686 and the 0.9 proof.

Alternatives considered

• Treat panic and allocator behavior as backend flags only. Rejected because packages, diagnostics, and target profiles need visible requirements.
• Require every freestanding artifact to provide an allocator. Rejected because many tiny proofs should be allocation-free.
• Defer this to the kernel proof. Rejected because allocator and panic assumptions should be resolved before QEMU boot work.

Scope / acceptance criteria

• In scope:
  • Experimental panic strategy metadata and validation.
  • Experimental allocator requirement metadata and validation.
  • Diagnostics for allocation/panic behavior incompatible with the selected target profile.
  • Build report output for panic and allocator facts.
  • Minimal hook plumbing needed by feature - no-std freestanding build mode and restricted artifact smoke test #686 and future 0.9 kernel proof work.
• Out of scope:
  • Stable public syntax for all panic/allocator policies.
  • General-purpose allocator implementation.
  • Full unwinding support on freestanding targets.
  • Production kernel panic subsystem.
• Done when:
  • A freestanding target profile can declare accepted panic and allocator strategies.
  • Code that requires unsupported allocation or panic behavior is rejected with a clear diagnostic.
  • Requirement reports expose the chosen panic and allocator strategy.
  • The restricted freestanding smoke test can use the hook model instead of hardcoded backend behavior.