t27a: mnemonic table and strict decoder for the 47 TRI-27 opcodes (sieve, T736) #6516

Boundary

• specs/isa/t27a_mnemonics.t27 (new file; nothing else)

User Scenarios

• A t27a assembler author needs one place that maps the 47 TRI-27 mnemonics' table index to the opcode byte and back, so the assembler and the disassembler read the same table (epic EPIC: golden reference language -- the compiler sieve (TNF-style theorems), and t27 passes it #6488, theorems T735-T736 in specs/compiler/theory/isa_round_trip.t27).
• A disassembler user must be told when a byte is not an opcode. Today decode_opcode in specs/isa/ternary_encoding.t27 maps the 209 undefined bytes to NOP, so garbage reads as NOP.

Requirements

• New module T27aMnemonics in specs/isa/t27a_mnemonics.t27, with use isa::ternary_encoding;.
• pub fn opcode_at(i: u32) -> u32 gives the opcode byte of table index 0..46 in the order of OPCODE_VALUES, and 256 for any other index. Write it as if chains, not array indexing.
• pub fn index_of(op: u32) -> u32 is its inverse, and gives 47 for an undefined byte.
• pub fn strict_decode(byte: u32) -> u32 returns the byte for a defined opcode and 256 (invalid) otherwise.
• Tests that call these functions at run time:
  • index_of(opcode_at(i)) == i for every i in 0..46;
  • exactly 47 bytes in 0..255 have strict_decode(b) != 256;
  • exactly 209 bytes are rejected;
  • a negative control showing decode_opcode maps byte 1 to NOP while strict_decode(1) == 256.
• Do not copy any function body from specs/isa/ternary_encoding.t27 or specs/compiler/theory/isa_round_trip.t27. The dupe ratchet rejects copied bodies; call them through use instead.
• Lines start with ; for prose (never a bare ;), ASCII only, English identifiers.
• The PR contains only .t27 and t27c-generated files (owner hard rule 2026-10-05).
• The PR says Closes #<this issue> and Refs #6488.

Success Criteria

• t27c typecheck specs/isa/t27a_mnemonics.t27 prints Typecheck OK (0 errors.
• t27c gen-c specs/isa/t27a_mnemonics.t27 > /dev/null exits 0.
• grep -c '^test ' specs/isa/t27a_mnemonics.t27 prints 3 or more.
• grep -c 'strict_decode' specs/isa/t27a_mnemonics.t27 prints 3 or more.
• git diff --name-only origin/master...HEAD lists only specs/isa/t27a_mnemonics.t27.