t27o: read-only vec![..] literals become array literals (step 1 of #10546) #10643

Goal

Step 1 of #10546 (lane V of epic #10522): a vec![a, b, c] or vec![x; N] (N a constant) that a fn only reads -- a local never pushed to, resized, mutated, moved on or returned -- converts to a typed t27 array, const v : [3]T = [a, b, c]; / const v : [N]T = [x] ** N;, which t27 passes as a slice. Anything the fn changes, hands on or returns, or whose element type t27o cannot tell, stays refused, with a reason that says which.

Boundary

• May touch: specs/tools/t27o/rust_convert.t27, its regenerated gen/rust/tools/t27o/rust_convert.rs, its seal.
• Must not touch: cli/tri/src/main.rs, spice_convert.t27, rust_std.t27.
• Stacked on the open t27o: a declaration index, a line cache and a bounded for-search make tri t27o linear in the file; compiler.rs finishes in 1.3 s (Closes #10544) #10631 (linear lookups) and t27o: integer ! is a bitwise ~, the oracle spec uses type_text, the harness drops no_std (Closes #10534) #10611, which edit the same spec.

Done when

• Tests per form (a local read by index, a local iterated with for, a local passed as &v, vec![0u8; 4]) and negative controls (a pushed vec, a returned or moved vec, an unknown element type stay refused) pass under t27b test, 0 vacuous under test-report.
• The macro vec first-refusal count is reported before and after on the same five repositories and commits, and no converted output changes except where vec! was involved.

User Scenarios

• Given a function contains a local let v = vec![a, b, c]; that is only read by index or iterated with for or passed as &v, and the element type is known, When t27o processes the function, Then the vec literal becomes a typed array const v : [3]T = [a, b, c]; passed as a slice.
• Given a function contains let v = vec![x; N]; with constant N that is only read, When t27o processes the function, Then the vec literal becomes const v : [N]T = [x] ** N;.
• Given a function contains a vec literal that is pushed to, returned, moved, resized, or has unknown element type, When t27o processes the function, Then the vec literal stays refused with a reason stating which condition prevented conversion.

Requirements

FR-001: t27o MUST convert a read-only vec![a, b, c] local with known element type to const v : [3]T = [a, b, c]; passed as a slice.
FR-002: t27o MUST convert a read-only vec![x; N] local with constant N and known element type to const v : [N]T = [x] ** N;.
FR-003: t27o MUST refuse conversion when the vec local is pushed to, and the reason MUST state "pushed".
FR-004: t27o MUST refuse conversion when the vec local is returned or moved, and the reason MUST state "returned or moved".
FR-005: t27o MUST refuse conversion when the vec local is resized or mutated, and the reason MUST state "resized or mutated".
FR-006: t27o MUST refuse conversion when the element type cannot be determined, and the reason MUST state "unknown element type".
FR-007: The macro vec first-refusal count MUST be reported before and after on the same five repositories and commits, with no converted output changes except where vec! was involved.