fix(html): make deeply nested input linear and stop nested-table row duplication

AGENTS.md

@@ -31,7 +31,7 @@ Fix root cause. Unsure: read more code; if stuck, ask w/ short options. Unrecogn
 | formats/format-support | Formats, bytes-in API, detection order, DOCX + spreadsheet mapping |
 | integrations/tool-contract | `MarkkitTool` LLM tool (`tool` feature), Everruns toolkit contract |
 | security/security-model | One-page threat/invariant summary |
-| security/threat-model | Threats T1-T9, mitigations, test map, resource limits, invariants S1-S20 |
+| security/threat-model | Threats T1-T9, mitigations, test map, resource limits, invariants S1-S21 |
 | runtimes/python-package | PyO3 `markkit` module, maturin, PyPI |
 | runtimes/wasm-package | wasm-bindgen `@everruns/markkit-wasm`, browser/edge, build + tests |
 | runtimes/node-package | NAPI-RS `@everruns/markkit`, npm, binding parity table |

CHANGELOG.md

@@ -152,6 +152,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/).
 
 ### Fixed
 
+- HTML: deeply nested input no longer costs quadratic time. 687 KiB of nested `<div>`s took
+  12.6 s and 292 KiB of unclosed nested tables ran past the 60 s timeout; tags nested past
+  `max_nesting_depth` are now dropped (text kept) before html5ever parses, so both take
+  milliseconds.
+- HTML: rows of nested tables are no longer copied into every enclosing table (output grew 4x
+  per nesting level).
+- HTML: text flattened past `max_nesting_depth` no longer includes `<script>`, `<style>`,
+  `<template>` and other non-content element bodies.
 - Spreadsheets: a sheet with a single row now renders that row as data under Excel column letters
   (`| A | B |`) instead of as a header with no rows.
 - Spreadsheets: `Extraction.title` is filled from document properties (XLSX `docProps/core.xml`,

crates/markkit/src/html/mod.rs

@@ -3,6 +3,7 @@
 //! Converts HTML input into the canonical IR.
 
 mod metadata;
+mod nesting;
 mod parser;
 mod readability;
 mod urls;