fix: apply pipe filters in SET/DEFAULT directives (GH #174)

[toddr-bot]

What

Pipe filters now work correctly with SET and DEFAULT directives.

Why

[% SET foo = "foo bar" | uri %] assigned "foo bar" unfiltered to foo, while
[% foo = "foo bar" | uri %] (without SET) correctly applied the filter. This
inconsistency has been open since 2010 (rt.cpan.org#59208, GH #174).

The root cause: SET forces parsing through setlist → assign → expr, but expr
doesn't include FILTER as an operator. The FILTER token was consumed by the outer
atomexpr FILTER lnameargs rule, wrapping the entire SET directive's output (empty
string) through the filter instead of filtering the assigned value.

How

Added filter-aware assignment rules scoped to SET and DEFAULT in the atomdir
grammar production. A new filter_expr() method in Directive.pm generates filter
code that returns the filtered value as an expression. Grammar changes are scoped to
avoid interfering with the capture: ident ASSIGN mdir path used by implicit assignments.

Testing

• New t/set-filter.t with 11 test cases covering SET, DEFAULT, implicit, expressions, and regressions
• Full test suite passes (3192 tests)

Closes #174

🤖 Generated with Claude Code

[toddr-bot]

Recreated from #380 (auto-closed when the toddr-bot fork was removed). Original branch is now hosted on this repo.

[toddr]

@toddr-bot rebase

[toddr]

@toddr-bot rebase

[toddr-bot]

Rebase with requested adjustments

Branch koan.toddr.bot/fix-set-pipe-filter was rebased onto main and review feedback was applied.

Stats

4 files changed, 4565 insertions(+), 4278 deletions(-)

Actions performed

• Already-solved check: negative (confidence=high, reasoning=No commit in recent main history addresses SET/DEFAULT pipe filter bug (GH explicit setting of a variable does not seem to work with pipe based filters [rt.cpan.org #59208] #174); none mention SET f)
• Resolved merge conflicts (1 round(s))
• Rebased koan.toddr.bot/fix-set-pipe-filter onto origin/main
• Pre-push CI check: previous run passed
• Force-pushed koan.toddr.bot/fix-set-pipe-filter to origin
• CI check enqueued in ## CI (async)

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