v0.5.1 — three post-audit fixes

Codemble v0.5.1 — three small fixes from a post-release audit

A full re-audit of the learner journey after v0.5.0 confirmed the big gaps were
closed, but turned up three smaller ones. This patch lands them. No new
features, no breaking changes: the CLI, the graph/checks API, and the on-disk
progress format are all unchanged.

Fixed

Cancelling a parse now stops the slow stage too

Returning to the picker during a parse used to stop only the file-reading loop.
The resolving stage — the slowest one on a large project — ran to completion
regardless, so a cancel there left a worker burning CPU, and starting a second
project could run two parses at once. Each resolving sub-step is now a
cancellation checkpoint, matching the per-file one, so a cancel is noticed
promptly at every stage.

Clicking a module on the 2D Map no longer looks like nothing happened

Easy mode opens on the 2D Map, where a module is a single box. Clicking one
selected it but drew no change, so the click looked broken. The selected box now
highlights in the interaction accent (never the amber that means "understood"),
and the system-level copy says plainly that the module's internal structures are
drawn as planets in the Galaxy layer, and that the Map shows how modules connect,
not what is inside them.

That copy is deliberately careful: it does not point the learner at the
Workflow tab, because a module the program never reaches has no rows there — a
promise the learner would have caught as wrong. Codemble never claims something
the view cannot show.

Known limits, unchanged

• Projects past roughly 1,000 supported source files still need scoping to a
  subdirectory; level-of-detail and clustering remain later-phase work.
• The layer choice (Galaxy vs Map) still returns to the mode default on reload —
  view state is intentionally session-only.