fix(interpreter): close Oils tilde, patsub, brace, loop and paren-ambiguity gaps

[chaliy]

Requested by Михайло · project thread

What changed

• Tilde expansion follows bash: only an unquoted ~ whose prefix ends at / or : expands (~"/b", \~, ~$v stay literal); assignments expand after = and each : (PATH=a:~/bin), also for name= arguments; ${x:-~} expands only unquoted. No user database: ~root → /root, ~<sandbox user> → /home/<user>.
• ${x/pat/rep}: replacement is literal after quote removal (${v/x/"?"} printed \?x); ${x////c} takes / as the pattern.
• Brackets: [C\-D] holds a literal -; reversed ranges [z-a] match nothing.
• Lexer: echo [bin; echo x no longer reads past the ;.
• Paren ambiguity: ((cmd) ...) / $((cmd) ...) parse as nested subshell / command substitution like bash.
• Loops: while break; do leaves the loop; ( continue ) only warns; continue 1 2 is "too many arguments".
• Brace expansion: unclosed { stays literal while later groups expand ({{a,b} → {a {b); step 0 counts as 1.
• ${v@op}: ${undef@Q} etc. expand to nothing, set -u rejects it, ${x@K} quotes a scalar.

Why

Level-up roadmap: next batch from the Oils harness ranking.

Before / After (verified)

Spec file	Before	After
tilde	9	1
var-op-patsub	9	2
var-op-bash	8	3
brace-expansion	8	4
glob	5	3
paren-ambiguity	5	0
loop	5	0
Full suite (bashkit vs bash)	77.3%	78.8%

Side gains: command-sub 2 → 0, builtin-eval-source 7 → 5. One case flips (background "Wait for job and PIPESTATUS"): bashkit gives the same output before and after; the old run counted it only because real bash's result for it is racy.

Spot check, identical in bash 5.2 and this branch:

HOME=/h; echo ~ ~"/b" '~' x=~      # /h ~/b ~ x=/h
y=a:~:b; echo $y                   # a:/h:b
v=axb; echo ${v/x/"?"}             # a?b
echo {{a,b} {1..5..0}              # {a {b 1 2 3 4 5
while break; do echo no; done      # (no output, loop exits)

Risk

• Expansion is a core path; new spec files (tilde-expansion, patsub-replacement, loop-control, paren-ambiguity) plus additions, all verified against bash. bash-oracle unchanged at 104/102.
• Stack for depth_32_no_stack_overflow: ~1,888,007 bytes vs 1,892,032 on the base; no new awaits in hot paths.
• Left: var-op-test (needs arrays / ${!ref}, owned elsewhere), bash's brace "double expansion" quirk, command-name globbing (would add an await to the hot path), globskipdots, LC_ALL=C byte matching. Recorded in limitations.

Checklist

• Failing tests first, verified against bash 5.2
• fmt, clippy, integration, lib, terminal, CLI, bash_comparison, check-okf, bash-oracle
• knowledge updated (log, limitations, oils-spec headline + saved run)

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