Phase 0 (#6656): software half of the incremental-update PoC -- static region, two compatible slots, sealed A -> B frontier

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Goal

Do the software/forge half of #6656 (parent EPIC #6655). It needs one static region, one reconfigurable slot with two compatible implementations, seals for each, and the A -> B step judged by specs/verified/frontier.t27 (#6673): the static region is reused, the slot is rebuilt. Hardware stays UNPROVEN.

Where we are

reuse.t27 (#6670, PR #6671) and frontier.t27 (#6673, PR #6674) define the rule. The repo has no partial-reconfiguration spec. The smallest clocked example is specs/ternary/clocked_counter.t27.

Boundary

Allowed: specs/verified/poc/** and the seals t27c seal --save writes for those specs (.trinity/seals/poc_PocStatic.json, a_PocSlot.json, b_PocSlot.json, poc_PocRun.json).
Out of bounds: the compiler, tri, specs/fpga/**, any bitstream, any device programming, any claim of a real partial-device update.

User Scenarios & Testing

• Given slot A and slot B, when t27c gen-verilog runs on each, then the module PocSlot (...) header is byte-identical.
• Given A -> B with the same toolchain, then the frontier reuses the static region and rebuilds the slot (REBUILD_SPEC).
• Given a different toolchain, then nothing is reused.
• Given the same injected test failure twice, then the same fingerprint, and the second sighting updates the issue rather than opening a new one.
• Given no device evidence, then the hardware state is UNPROVEN.

Requirements

• FR-001: Pure .t27; every observation is produced by t27c seal or t27c gen-verilog, not typed by hand.
• FR-002: No hardware claim.

Success Criteria

• t27c seal <spec> --verify prints all hashes MATCH for static_counter, a/slot, b/slot and run.
• t27c gen + zig test passes for each of the four specs; run.t27 prints All 4 tests passed.
• The module PocSlot ( ... ); blocks of the gen-verilog output for a/slot and b/slot differ by zero lines (diff exit 0).
• spec_hash in a_PocSlot.json differs from b_PocSlot.json.
• run.t27 asserts hw_state(true, false) == HW_UNPROVEN.