Preserve text after an ignored form in a dropped table

[jmanico]

Summary

• use the existing output-context/form-pointer seam when the input foreign-content tracker is unknown and the output parser would ignore a second HTML form start
• discard that browser-ignored start without leaving a logical policy form, while marking the nearest missing table unavailable so later table parts cannot serialize without their table
• preserve the exact trailing z reproducer and add hostile SVG, MathML, nested-table, template/caption, explicit-end-tag, and active-content regressions

The exact reduced input now changes from:

<svg><foreignObject><form id="a">y</form></foreignObject></svg>

to the stable output:

<svg><foreignObject><form id="a">yz</form></foreignObject></svg>

Addresses #492, item 7. Item 3 remains open.

Verification

• differential sanitizer probe against exact base 111c010dfc3294e41ca56a4d118a7780c26c1551: 15,430 inputs × 44 policies = 678,920 rows, zero gained flags of any kind (including non-idempotent, tree-mutates, text-in-*, and exceptions)
• differential limit/event probe: 36,063 rows, zero gained flags and zero changed outputs
• base and head used the same compiled probe (f56c346716f848914be899f37b1b422c1a84b113a97526b71691ca76e33ba972 source hash), corpus, policies, and eight seeds
• ./mvnw -o -ntp -B clean verify (693 library tests, 7 example tests, JPMS consumer check)