Make surface JIT DEM boundary providers data-driven

[qciaran]

Follow-up from #664 and its review.

The PR now shares boundary placement and canonical two-patch coordinate translation across the H, CX, and mixed H/CX providers. The remaining providers still encode eligibility checks, fixture selection, output routing, coordinate placement, schedule construction, and fallback decisions in separate methods. Adding a logical-gate family therefore still risks copying a substantial amount of orchestration code.

Acceptance criteria:

• Define a typed boundary-family/provider description covering eligibility, canonical fixture keys, template selection, detector placement, GF(2) output routing, and exact-compile fallback conditions.
• Migrate the H, CX, and mixed H/CX providers without changing their bounded cache keys or observable-reliability behavior.
• Share schedule construction and common validation while keeping gate-specific physical template compilation explicit.
• Ensure the assembled-circuit output schema remains the independent authority rather than provider-local reliability rules.
• Add table-driven tests for one and repeated boundaries, mixed schedules, coordinate relocation, incompatible geometries, and fallback cases.
• Document the minimum implementation required to register a new logical Clifford boundary family and measure the resulting code reduction.

[qciaran]

Implemented in PR #664 at 9bb8784. H, repeated CX, and mixed H/CX now produce typed provider descriptions pairing eligibility/fallback guards, canonical fixture keys and template selections, detector placement, and GF(2) routing policy. One common assembler owns placement, independent assembled-output schema validation, schedule construction, and stitching. The full provider test file passes (65 tests), including table-driven routing and existing exact/fallback/relocation coverage.