RFC3: Move Delay value storage to a typed arena

[ihincks]

This PR is one of three RFCs solving the same problem: dt-unit delays panic in qk_circuit_get_instruction because their integer duration is stored as Param::Obj(PyAny).

This branch takes the most surgical fix. Delay durations leave the Param list entirely and move into a typed global arena (crates/circuit/src/delay_arena.rs). The arena holds a DelayDuration enum (Int, Float, Expr, PyObj) inside refcounted Slots, and StandardInstruction::Delay(DelayHandle) replaces StandardInstruction::Delay(DelayUnit). Standard gates and other instructions are untouched — only the Delay variant knows about durations now.

The C API gets qk_circuit_delay_dt(qubit, i64) and qk_circuit_get_delay(idx, *unit, *kind, *vi, *vf) with a CDelayDurationKind enum that distinguishes integer, float, and expression durations. QPY format is unchanged — the writer synthesizes a duration Param from the handle when it serializes a Delay, and the reader builds a fresh handle from param[0] on read.

The trade-off is implementation complexity. StandardInstruction loses Copy and gets a manual PartialEq. PackedOperation::clone and drop special-case the Delay case to bump and release the refcount. The Box<Slot> indirection in the arena is load-bearing — with() snapshots a *const Slot then drops the read guard. There's a new ParameterUse::DelayDuration variant for symbolic durations that the parameter table needs to track separately.

The other two RFCs are #16226 (a minimal int → float coercion patch) and #16241 (a uniform Param::Int variant that touches every match over Param).

Partially addresses #16121.

AI/LLM disclosure

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[coveralls]

Coverage Report for CI Build 26299637026

Coverage decreased (-0.03%) to 87.461%

Details

• Coverage decreased (-0.03%) from the base build.
• Patch coverage: 148 uncovered changes across 13 files (453 of 601 lines covered, 75.37%).
• 17 coverage regressions across 7 files.

Uncovered Changes

Top 10 Files by Coverage Impact	Changed	Covered	%
crates/circuit/src/circuit_data.rs	78	46	58.97%
crates/transpiler/src/passes/instruction_duration_check.rs	19	1	5.26%
crates/qasm3/src/exporter.rs	22	5	22.73%
crates/cext/src/circuit.rs	79	63	79.75%
crates/qpy/src/circuit_reader.rs	42	29	69.05%
crates/circuit/src/circuit_instruction.rs	45	33	73.33%
crates/transpiler/src/passes/constrained_reschedule.rs	11	0	0.0%
crates/circuit/src/delay_arena.rs	190	180	94.74%
crates/accelerate/src/circuit_duration.rs	21	13	61.9%
crates/circuit/src/circuit_drawer.rs	18	12	66.67%

Coverage Regressions

17 previously-covered lines in 7 files lost coverage.

File	Lines Losing Coverage	Coverage
crates/qasm2/src/lex.rs	6	91.77%
crates/circuit/src/operations.rs	4	82.21%
crates/accelerate/src/circuit_duration.rs	3	72.88%
crates/circuit/src/circuit_data.rs	1	86.06%
crates/circuit/src/parameter/parameter_expression.rs	1	91.04%
crates/circuit/src/parameter/symbol_expr.rs	1	73.88%
crates/qasm3/src/exporter.rs	1	79.18%

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Coverage Stats

Relevant Lines:	124345
Covered Lines:	108753
Line Coverage:	87.46%
Coverage Strength:	963161.1 hits per line

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💛 - Coveralls