fix(tdx-hydro): widen guarded reach ambiguity band

[CooperBigFoot]

Summary

• widens only the guarded non-root reach-side ambiguity pre-pass from two to three endpoint tolerances
• preserves the 0.001 endpoint matching tolerance, the independent two-tolerance indeterminate-root refusal, and unconditional conflicting-root refusal
• adds motivating, outer-bound, reversed-orientation, area-guard, and root-bound regression coverage
• documents the distinct non-root and root limits

Local review

• write set: adapters/tdx-hydro/build_adapter.py, adapters/tdx-hydro/test_build_adapter.py, and adapters/tdx-hydro/README.md
• scope fences: no version, changelog, specification, schema, validator, conformance, or repository-contract changes
• pre-image sensitivity: all six affected fixture tests fail with the unchanged cutoff; four encounter the old 0.002 refusal instead of their expected downstream behavior, the outer-bound message reports 0.002 instead of 0.003, and the root-bound tuple lacks the new named multiplier
• targeted changed-fixture probe: passed (6 tests)
• critic review: passed with no findings; confirmed only the non-root site changed, the root and conflict branches are byte-for-byte unchanged, all fixture mutations are sensitive, exact diagnostics match, and README wording is accurate

Acceptance gates

• cargo fmt --check: passed
• cargo clippy --workspace --all-targets -- -D warnings: passed
• cargo check --workspace --all-targets: passed
• cargo test --workspace: passed
• cargo build --workspace: passed
• adapter module with debug HFX binary: passed, 96 tests; the first run exposed a transient unavailable nested check-jsonschema tool, and the identical full command passed after that ephemeral tool resolved

Remaining risks

• The frozen 55-basin campaign was intentionally not recompiled; this local change relies on authored synthetic coverage and the full adapter module.