fix(lang): key the removed-phase entry by the name that was removed

[haribo]

Summary

removedPhases turns a retired phase name into an actionable error. Its first entry was keyed
"check" — a valid phase, matched one case earlier in the same switch
(def_parser.go:78 before :80) — so the entry was unreachable, and pre-check, the name it
was written for, got the generic expected a phase, got "pre-check".

Keyed "pre-check" now, with the reason on the map so the next reader does not re-derive it.

The sharper half: a test that could not fail

TestPhases_RemovedNamesRefused asserted strings.Contains(err.Error(), "check") for input
pre-check. The generic message contains the input, so it contained check: the test passed
with no removedPhases entry at all, which is exactly the state it was in.

Both wanted substrings now appear only in the actionable message — rename the phase and
removed (ADR-0035). The post case was already sound ("removed" is absent from the generic
message); it was checked rather than assumed, as the issue asked.

Test plan

Mutation-tested in both directions, on the corrected assertion:

drop the pre-check entry     → FAIL  control_test.go:456: pre-check: expected a phase, got "pre-check"
re-key it "check" (the bug)  → FAIL  same
restored                     → ok

The four surviving phases still parse (same test). go test ./... and test/lint.sh green.

Closes #637