fix(planner): preserve unavailable all-target plans

[szmyty]

Context

Renderflow's post-#407 main run failed only test_all_without_transforms_uses_builtin_capability_registry: a clean Ubuntu host pruned every optional --all target by provider availability, then aborted before emitting the artifact-forest plan.

Changes

• partition policy-allowed targets from provider-available targets explicitly;
• retain unavailable-only --all branches solely to construct an inspectable dry-run plan;
• keep those artifact-forest branches classified as unavailable;
• preserve fail-closed execution preflight before any transform step or output;
• make the clean-host regression deterministic with an empty PATH;
• add pure coverage for mixed availability, zero availability, and required exact-target fallback;
• add non-dry-run evidence for a failed run with zero steps and zero outputs;
• correct CLI, overview, landing, and troubleshooting docs that still claimed graph planning always requires a transforms file.

Architecture impact

The planner now carries three distinct sets through resolution: requested/policy-allowed targets, provider-available targets, and pruned unavailable targets. Only provider-available targets are marked selected in the artifact forest. Unavailable targets are used as a planning fallback only when every branch is unavailable; the executor's existing provider preflight remains the authority and fails before work begins.

Provider registration, availability, policy permission, and selection remain separate claims. No schema, workflow, provider implementation, or sibling contract changed.

Pinned starting point

• main: 1eafaf54d33a984377a7a3e327ee87382da73307
• failing main run: https://github.com/egohygiene/renderflow/actions/runs/34775426178
• final PR head: 9a83038fd90f82c49ddf438497445c59a116abe1
• final tree: edbaf6c6a87fcb03e3c8cd600822dd9bbc666358
• two commits, six focused files, zero commits behind the pinned base
• no open Renderflow PRs at the pre-implementation check
• dependency changes: none

Validation

All final-head hosted gates are green:

• CI: https://github.com/egohygiene/renderflow/actions/runs/34905427915
• generated docs/schema and strict versioned preview: https://github.com/egohygiene/renderflow/actions/runs/34905427909
• artifact conformance: https://github.com/egohygiene/renderflow/actions/runs/34905427924
• commit lint: https://github.com/egohygiene/renderflow/actions/runs/34905427907

Those successful runs covered strict Clippy, build, complete Rust tests, both AUR package checks, Snap build/install, portable installer smoke, bundled dry-run and embedding examples, web format/lint/typecheck/tests/build, generated-doc/schema drift, strict MkDocs/Mike preview, and the hermetic conformance corpus.

Local validation included git diff --check and exact remote-tree comparison. This workspace does not expose a Rust toolchain, so the repository-pinned hosted jobs are the executable validation authority.

Rollout and review

The PR is intentionally unmerged. It references rather than auto-closes #410 so the issue can remain open through review, merge, and the required green resulting-main CI run. If resulting-main differs from the PR evidence, keep #410 open and investigate rather than retrying or weakening a gate.

Refs #410
Relates to #407
Relates to #367
Relates to egohygiene/flow#11

Roadmap-Step: REN-Q02

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