t27b: every plan module self-built, hand glue to zero, fixed point stage2 == stage3

Steps 3d-4 of the matryoshka (see #8920):

(1) every specs/tri/t27b/*.t27 module is self-built like step 3c;
(2) hand-written glue in cli/t27b reaches zero (see epic: t27b in t27 -- port the backend and its tooling from Rust/Python into .t27 #6198);
(3) the front end comes from t27core (see epic: self-host t27c -- rewrite the compiler from Rust into t27 #5980 P4/P5), not t27c's Rust;
(4) the fixed point holds: stage1 = t27b built by the seed, stage2 = built by stage1, stage3 = built by stage2, and stage2 and stage3 are bit-identical;
(5) DDC (diverse double compiling) with an independent route reaches the same bytes (see verified: a signed DDC receipt of t27core and T732's verdict over its routes (Closes #7699) #7706).

Tracking issue; the work is weeks.

User Scenarios

Given the t27b seed compiler and specs/tri/t27b/*.t27 modules, when the matryoshka build runs steps 3d-4, then every module is self-built, hand-written glue in cli/t27b is eliminated, the front end comes from t27core, and stage2 and stage3 outputs are bit-identical.

FR-001: Every module in specs/tri/t27b/ MUST be self-built by the t27b compiler produced in the previous stage.
FR-002: Hand-written glue code in cli/t27b MUST reach zero; all glue MUST be generated from specs.
FR-003: The front-end compiler logic MUST be sourced from t27core (per epic: self-host t27c -- rewrite the compiler from Rust into t27 #5980 P4/P5), not from t27c's Rust implementation.
FR-004: The fixed-point property MUST hold: stage2 (built by stage1) and stage3 (built by stage2) MUST be bit-identical.
FR-005: Diverse double compilation (DDC) via an independent route MUST produce byte-identical artifacts to the primary build (per verified: a signed DDC receipt of t27core and T732's verdict over its routes (Closes #7699) #7706).

python scripts/tri_loop/t27b.py --verify-fixed-point exits 0 and reports stage2 == stage3 bit-identical.
python scripts/tri_loop/t27b.py --verify-ddc exits 0 and reports DDC artifacts match primary build.
grep -r "hand.*glue" cli/t27b/ returns 0 matches (exit 0 with empty output).
5 tests in the t27b self-build suite pass, each verifying a specs/tri/t27b/*.t27 module builds itself.