Tag balancer: five foster-parenting and text-gate gaps present on main

Five balancer behaviours found while reviewing #490 that are present on main (569d463) today
and are not changed by that PR. Each was reproduced on main, the #490 base, and the #490 head;
none is an XSS, and none needs an advisory. They are here so they are not lost.

1. An implied <li> lands inside a foreign root and flips a child's namespace. With a policy
   allowing ul, li, svg, textarea: <ul><svg><textarea>x</textarea></svg></ul> →
   <ul><svg><li><textarea>x</textarea></li></svg></ul>. The browser's tree of that output is
   ul > {SVG}svg > li > textarea — li is an HTML breakout name, so the textarea becomes an
   HTML textarea rather than the SVG element the input had. Same with <a> and <mi>. The
   containment metadata has no entry for foreign content, and text inside a foreign element under
   a ul still gets the li the metadata implies for ul. Skipping HTML containment for foreign
   text is not safe as things stand: the policy's text gate treats a foreign tbody as an HTML
   one and would drop D in <ul><svg><tbody>D.

2. Sanitizers.TABLES emits an orphan table part that is not a fixed point.
   <template><ul><math><caption>TEXT → <caption>TEXT</caption>; a second pass gives
   <table><caption>TEXT</caption></table>, and a browser parsing the first output drops the
   caption element entirely, leaving bare text. Same with <svg> for <math> and <colgroup>
   for <caption>. A published prepackaged policy.

3. disallowTextIn is bypassed for text foster-parented out of a retired table.
   new HtmlPolicyBuilder().allowElements("table","tbody","tr","td","form").disallowTextIn("form")
   on <tbody><form>B emits B (beside the table). The gate follows the nearest kept element,
   and when the table's output is retired for the text, the element the author disallowed text in
   is no longer the one judged. main suppresses it only because it never retires the table
   there and keeps the text inside the form, which is the #484 bug.

4. openDocument() does not reset the stacks; only closeDocument() does. A caller whose
   receiver throws mid-document never reaches closeDocument(); the next openDocument() then
   starts with the previous document's openElements and the fourteen parallel structures, and
   the second document's output begins with the first document's close tags:
   </tr></tbody></table><svg><path>y</path></svg>. PolicyFactory builds a fresh balancer per
   call so it is unaffected; direct users of the public class are not.

5. A foreign root with content inside a select grows on every pass.
   <select><svg>x</svg></select> → 38, 47, 56, 65 characters over four sanitizations, one
   <ul><li> level per pass, until the nesting limit truncates it. Same for <math> and for
   <select><table><svg>x</svg>. Repeated sanitization of stored HTML (edit/save cycles) grows
   the document without bound.

From the review comments on #490.

Left over from the third review of #490, accepted for merge (2026-09-18)

These are behaviours of the #490 branch after its fix series (eb11921..6c1fb9f). Unlike the
five above, items 6 and 7 are not on main, which keeps the text; they were accepted for the
merge because each needs several unusual things at once and none is an XSS. Item 8 has a
sibling on main. Item 9 is on main too. Reproductions and the probe are in the local
probes-review3 folder referenced from the PR comments.

6. Text after an emitted breakout inside a lexically open foreign root is dropped by a
   deliberate fail-closed rule. <svg><tbody><table><foreignObject><form>tail → the word
   tail is gone (main: <svg><tbody><table><foreignObject><form>tail</form>…). A browser
   pops the svg when the table breaks out, but the sanitizer writes the table inside the
   svg, and the policy's rule for text in a suppressed table part fails closed once the
   output tracker has no foreign root left. The rule is intentional; the open question is
   whether it should apply here, or whether the balancer should close the foreign root when it
   emits a breakout. Same class: one random probe input under the table-dropping and
   table-renaming policies where a stale <b> used to slip the text past the rule.

7. A second, pointer-ignored form in a row of a dropped table inside an integration point
   loses the text after the table. Policy drops table, keeps form/tr/td:
   <svg><foreignObject><table><form id=a><tr><form id=r><td>y</td></tr></table>z →
   <svg><foreignObject><form id="a">y</form></foreignObject></svg>; z is gone (main keeps
   both). Without the second form, or outside the integration point, z is kept.

8. A list item after a resumed formatting element nests inside the previous item.
   <math><b>x</math><li>y<li>z under Sanitizers.BLOCKS.and(FORMATTING) →
   <b>x</b><ul><li><b>y<ul><li>z</li></ul></b></li></ul>, a browser puts the two items side
   by side, both bold. Stable output. main does the same for <ul><li><b>x<li>y: the resumed
   <b> hides the open li from the next <li> start, which should close it.

9. An option under a dropped table cell is emitted without its select and is not a fixed
   point. Policy drops the table parts: <table><th><option>tail → <option>tail</option>,
   and the next pass gives <select><option>tail</option></select>. On main, 33c2b90 and
   the fixed head alike. On the fixed head this surfaces in about twenty random probe inputs
   that a queued nobr/b used to mask by being resumed around the option, with the select
   implied inside it.

From the third review comment and the fix-series comment on #490.

Start by reproducing the five main cases against commit 569d463 and inspect the balancer's containment, text-gate, and document-stack paths, including openDocument() and closeDocument(). Compare the accepted review cases with the referenced probes-review3 folder, then separate the behaviors into focused fixes. Done means the listed namespace, fixed-point, text-filtering, reset, and repeated-sanitization failures no longer reproduce, with regression coverage for each.