feat(tri27): SHA-256 + matrix multiply in .t27 assembly (refs #474)

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Summary

Two new TRI-27 assembly programs for TTT Dogfood Phase 3:

SHA-256 (src/tri27/sha256.t27)

• Full SHA-256 compression of a 64-byte message block
• Message schedule initialization, working variables, round compression
• Test vector: SHA-256("abc") = ba7816bf...
• Memory layout: msg_block, W schedule, a..h working vars, H[0..7] state

Matrix Multiply (src/tri27/matmul.t27)

• 3x3 matrix multiplication with triple-nested loop
• C = A * B using accumulator pattern
• Verification: [[1,2,3],[4,5,6],[7,8,9]] * [[9,8,7],[6,5,4],[3,2,1]] = [[30,24,18],[84,69,54],[138,114,90]]

Refs #474

Test plan

• Load and execute on TRI-27 VM
• Verify SHA-256 matches known test vectors
• Verify matmul result matches expected values