## Why (from the #127 trial-build gate)
The gate did its job: 0/3 passed, merge blocked. The log showed *why* -- BFpack, BayesERtools, GMLTM all fail while building their shared dependency **`rstan`**, not in their own code:
```
Failed to build source package rstan.
.../StanHeaders/include/stan/math/prim/core/init_threadpool_tbb.hpp:9:10:
fatal error: tbb/tbb_stddef.h: No such file or directory
```
So the per-package `-DTBB_INTERFACE_NEW` makevars entries the classifier proposed are useless for these packages -- they're blocked on `rstan` (which already has a registry entry). This is the **same dependency cascade** the RcppParallel `applies_to` guard catches, but `tbb-stddef-removed` is a generic signature with no such pin, so ~73 Stan packages kept getting proposed.
## What
Generalise cascade detection beyond the RcppParallel special case:
- `failing_dependency(error_text, package)` -- when the log names a **different** package as the one that failed to compile (`Failed to build source package X`, `compilation failed for package 'X'`, `dependency 'X' ... not available`), that package is the real cause.
- `build_triage_report()` now blocks any package whose **every** failing build is such a cascade: reported as `blocked_on` that dependency, never proposed a bogus per-package entry. A package that fails in its **own** compilation is still proposed.
- The `applies_to` (RcppParallel) and data-driven (rstan) cases are unified into one `blocked_packages` / `blocked_on` model; the report, proposer, and `blocked_summary` count the actually-blocked packages, and the blocked note shows even when a group also has genuine proposals.
## Effect
Next auto-apply run will stop proposing the rstan-blocked Stan packages (and any future dependency cascade) and surface them as "blocked on rstan" instead. Fixing `rstan` once clears the whole cluster.
## Verification
- New tests: `failing_dependency` (cascade vs own-compile vs none), and an end-to-end split where BFpack/GMLTM (blocked on rstan) are not proposed while an own-compile package still is.
- Full suite: 112 tests pass; all pre-commit hooks pass.
Refs #120, #127. (Separate follow-ups: fixing rstan's build itself, and quieting the gate's metadata-DB retry storm -- both root-caused to bincraft.)
Reviewed-on: #128
pat-s
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