fix(completion): flag-aware partial so a dash completes flags, not keywords

The partial-token walk stopped at `-`, so after typing `-` (or `--`)
the partial was empty and the replaced range was a zero-width point
*after* the dash. Two bugs followed at a flag position (e.g.
`add 1:n relationship … -`): the `on` keyword was offered (it
prefix-matched the empty partial), and accepting a candidate inserted
after the dash — `-on`, `---create-fk`, `----all-rows`.

Detect a dash-prefixed token at a word boundary as a flag-in-progress
and fold the whole dash-run into the partial, gated on a flag actually
being expected there (so `where x = -5` stays a signed number, not a
flag). The flag matcher now strips leading dashes and matches the body
uniformly (empty / `-` / `--` → all flags; `--cr` → create-fk).

Keywords like `on` no longer appear after a dash, and accept replaces
the dash(es) so `-` → `--create-fk` and `--all` → `--all-rows`. Two
partial-flag snapshots updated (they had captured the old behaviour).

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__delete_all_rows__delete_partial_flag_is_incomplete@partial_flag.snap

@@ -24,10 +24,10 @@ Assessment {
     completion: Some(
         Completion {
             replaced_range: (
-                24,
+                22,
                 27,
             ),
-            partial_prefix: "all",
+            partial_prefix: "--all",
             candidates: [
                 Candidate {
                     text: "--all-rows",

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__update_all_rows__update_partial_flag_name_is_incomplete@partial_flag.snap

@@ -24,10 +24,10 @@ Assessment {
     completion: Some(
         Completion {
             replaced_range: (
-                33,
+                31,
                 36,
             ),
-            partial_prefix: "all",
+            partial_prefix: "--all",
             candidates: [
                 Candidate {
                     text: "--all-rows",