fix(hook): stop four shapes from closing a live block

[edouard-claude]

Closes the first four shapes of #138. The fifth (a trailing backslash line continuation) has a different cause and is left for its own PR.

The defect

The command-position approximation read a reserved word that is not a command as one, popped a block that was still open, and the body was then rewritten. A consumer of the block read snip's compacted output instead of the real one. All four corrupt identically on master.

Reproduced end to end against real bash, 60-line file behind filters/grep.yaml's cap of head n:50:

shape	raw	before	after
case $s in done)	60	51	60
for f in @(data|done).txt	60	51	60
time -p for i in 1	60	51	60
a)# done marker	60	51	60

The fixes

• Case-arm patterns. blockScope tracks the arm state of a case: entered at the in that ends the subject, re-entered at each ;;, left at the arm's ). Words read in that state are patterns, so done) no longer pops the case. esac is the one exception, because bash rejects a bare esac as a pattern; without it the scope would stay open and everything after it would lose filtering. ;; is reported by the segmenter, since ';' is a group boundary and advance never sees one.
• Extglob alternatives. Inside a paren that is not a subshell, '|' is pattern alternation and no longer restores command position.
• time options. -p and -- keep command position, so the opener being timed is still seen. Bare time is unchanged.
• )#. A '#' flush against an operator ')' opens a comment. advance decides this where the kind of the paren being closed is known, which distinguishes it from $(ls)#tail where the ')' ends a command substitution and the '#' is still inside the word. The segmenter's own isWordStart is deliberately untouched: adding '{' there would read ${#var} as a comment.

The file-level comment listing these four as known limitations is updated in the same commit.

Tests

TestRewriteCommandPosition covers the four shapes, the ;; re-arm on a later arm, and five non-regressions: $(ls)#tail, ${#f}, a subshell )#, bare time, and filtering resuming after esac. Every case fails on master.

make test-race, make verify (55/55) and golangci-lint are clean.