Treat unspaced & as a background operator unless redirect-bound

[TheGreatAxios]

Fixes CL-7781: a permission bypass where & with no trailing space hid a second command inside one approved segment.

Previously isRedirectAmpersand keyed off the character following &, so a &b was treated as a redirect token and stayed one segment — a standing grant for the benign head auto-covered the hidden payload. Now & is redirect-bound only by preceding context (previous non-space char is >/<", e.g. 2>&1, >&2, <&-") or an immediately following > (&>/&>>). Everything else falls through to the existing chain-boundary branch, so a &b splits into two segments while bun run build 2>&1 stays unfragmented.

Tests:

• splitChainedCommand segment-count table: a &b, a & b -> 2; a &>f, a 2>&1, a >&2, a <&- -> 1; a&&b, a &&b -> 2
• Gate regression: standing grant for bun test * does not cover bun test x &touch pwn
• bun run check: green (6990 pass, 0 fail)

Sibling-operator audit (same loop): &&, ||, |, ;, newline need no preceding-context rule — they are unconditionally control operators and can never appear inside a redirect token (redirects are built only from digits, >, <, &, -). The lone & was the only operator with dual redirect/control use. Noted pre-existing quirk (unchanged by this fix, out of scope): |& drops the & but still splits, so nothing hides. Heredoc regions untouched (shell-serial branch owns them).

Related: CL-7781

[linear-code]

CL-7781