refactor(preg): report unmatched groups as null throughout

The shim carried two reporting modes for the preg_* $matches maps: PHP's
default (trailing unmatched groups dropped, interior ones ""), and the
PREG_UNMATCHED_AS_NULL form, picked by calling a *_unmatched_as_null()
variant. The regex crate hands out Option<Match>, which maps onto the
null form directly, and no caller distinguished a dropped group from a
null one -- preg_match() and preg_match_all2() already reported nulls
unconditionally. Keep only the null form; the shim API no longer mirrors
PHP's flag set, which is intended.

Preg::is_match_with_indexed_captures() modelled PHP's "unset" as a
truncated Vec<String>, and now returns Vec<Option<String>>. That is what
Composer actually does: Preg::isMatch() always sets
PREG_UNMATCHED_AS_NULL, and its callers test groups with `!== null`.

preg_match_all(), preg_match_all_set_order() and
preg_split_delim_capture() still hand back Vec<String> and keep the ""
form.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

2 files changed, 2 insertions, 4 deletions

diff --git a/crates/shirabe/src/util/hg.rs b/crates/shirabe/src/util/hg.rs
index 8570af6d..d9e58623 100644
--- a/crates/shirabe/src/util/hg.rs
+++ b/crates/shirabe/src/util/hg.rs
@@ -158,7 +158,7 @@ impl Hg {
                         &output,
                     )
                 {
-                    return matches.into_iter().nth(1);
+                    return matches.into_iter().nth(1).flatten();
                 }
                 None
             })
diff --git a/crates/shirabe/src/util/platform.rs b/crates/shirabe/src/util/platform.rs
index 21ad869c..629854b6 100644
--- a/crates/shirabe/src/util/platform.rs
+++ b/crates/shirabe/src/util/platform.rs
@@ -95,14 +95,12 @@ impl Platform {
         // Regex pattern compatibility:
         // The original pattern uses a conditional subpattern to make the trailing `%` required
         // only for the `%VAR%` form. The Rust regex crate does not support conditionals, so the
-        // two forms are written as an explicit alternation: `$VAR` or `%VAR%`. The branch that did
-        // not participate is reported as an empty string, which `\w+` can never capture.
+        // two forms are written as an explicit alternation: `$VAR` or `%VAR%`.
         Preg::replace_callback(
             php_regex!(r"#^(?:\$(?P<dvar>\w+)|%(?P<pvar>\w+)%)(?P<path>.*)#"),
             |matches: &PregMatchedGroups| -> String {
                 let var = matches
                     .get(&CaptureKey::ByName("dvar".to_string()))
-                    .filter(|dvar| !dvar.is_empty())
                     .or_else(|| matches.get(&CaptureKey::ByName("pvar".to_string())))
                     .map(|s| s.as_str())
                     .unwrap_or("");