feat(tools): read a fork's own manifest keys and its fork_core additions

[braboj]

Part of #1944 (a prerequisite the split exposed).

The gap. After the fork merges the split with -s ours, about 33 moved templates sit under base: and backend:, and base-readme sits in core:. ADR-049 rule 4 and SYS-20 forbid a fork to edit those keys. Rule 4 already lets a fork append "keys of its own", but resolve.py and smoke read five fixed keys, so a fork's template under its own key resolved as a missing dependency. A fork had no legal place to keep its templates.

Change:

• resolve.py gains entry_sections(manifest), which returns every top-level list of entries except version, core and fork_core, and core_tier(manifest), which returns core followed by a fork's fork_core. load_manifest uses both.
• run_smoke.py imports the same two functions for _load_manifest, SYS-03 and SYS-04, so there is one definition of a section.
• SYS-21: resolves a planted manifest with an acme: key and fork_core: through the resolver's CLI, and asserts the chain carries the upstream core file, the fork's dependency and the fork's core addition. SYS-19's loop is extracted into _resolve_cases and shared.
• ADR-049 rule 4 is amended in place (pre-release, as before): "…and adds to every chain's core tier under fork_core:". The SPEC core-tier section and the manifest header say the same.

Control: with main's resolve.py, the SYS-21 manifest exits 1 with Missing dependency: stack-fork depends on fork-lib, which has no manifest entry. With this branch it resolves quality.md, docs.md, git.md and the stack.

Test changes: SYS-19's assertions are unchanged; only its loop moved into a helper. SYS-21 is new.

Local (Git Bash): smoke 36/36, conformance 24 / 0 failed, fork sync 3/3 (nested smoke 36/36), sync, resolve and redundancy clean. Upstream's own manifest has no fork key, so every chain is unchanged (resolve.py --check clean).

🤖 Generated with Claude Code