feat(server): Wave 6 -- source/understand pagination hardening (items a-e)

Audit follow-up (2026-08-23), final wave of the 6-wave plan.

- item a: source()'s edit_symbol suggestion no longer says "no preview
  needed" when the response was truncated -- the etag/expected_hash is
  still valid (always hashes the FULL range), but only part of the body
  was actually seen; the reason text now says so and points at
  resume_from_line.
- item b: understand()'s embedded source_output now honors max_lines/
  resume_from_line via the same paginate_range() source() uses, instead
  of always returning the whole symbol body unpaginated.
- item c: max_lines/max_chars <= 0 are now rejected with INVALID_PARAMS
  at the tool boundary (source() and, via the same check, understand())
  instead of silently falling back to "unlimited".
- item d: new max_chars param (SourceParams + UnderstandParams) hard-caps
  rendered text length on top of whatever max_lines already selected --
  counts whole lines only, never splits one line's own characters across
  pages (apply_char_budget/narrow_by_char_budget, shared by source(),
  source_range(), and understand()).
- item e: resuming a paginated source() read with a stale if_none_match
  (the range changed since that etag was issued) now returns a new
  RANGE_CHANGED_SINCE_PAGINATION error (needs_verification) instead of
  silently serving a page sliced against new bytes at old coordinates.
  The pre-existing not_modified shortcut is now gated on
  resume_from_line being unset, so it no longer swallows a legitimate
  page-2+ request that happens to carry a still-valid etag.

10 new/changed tests (5 new: truncated-suggestion wording, max_lines/
max_chars <= 0 rejection, max_chars narrowing, stale-if_none_match
rejection, understand() pagination; plus SourceParams/UnderstandParams
literals updated at every existing call site for the 4 new fields).
1291 calm-core + 465 calm-server tests green, clippy/fmt clean.

Dogfooding note: hit a reproducible false-positive PARSE_ERROR from
edit_lines/edit_symbol specific to re-editing understand()'s tail a
second time this session, on trivially-valid Rust (confirmed via direct
cargo check + isolated sub-edits). Worked around via one native Edit
for that single hunk, with explicit user sign-off; everything else in
this wave went through CALM's own edit tools as usual. Worth a root-cause
pass on CALM's own edit-validation path separately.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: Your Name

crates/calm-server/src/__toolsnaps__/source.snap

@@ -73,6 +73,14 @@
           "null"
         ],
         "format": "int64"
+      },
+      "max_chars": {
+        "description": "Wave 6 (item d): hard character cap on the rendered `source` text,\napplied on top of whatever `max_lines` already selected (never\nwidens a page, only narrows it further) -- counts whole lines only,\nnever splitting a single line's own characters across pages. Known,\naccepted limitation: a single line alone longer than `max_chars` is\nstill returned whole (can't be sub-divided by byte offset), and\n`next_cursor` on the following page points straight back to that\nsame line rather than skipping past it.",
+        "type": [
+          "integer",
+          "null"
+        ],
+        "format": "int64"
       }
     },
     "type": "object"

crates/calm-server/src/__toolsnaps__/understand.snap

@@ -18,6 +18,30 @@
           "string",
           "null"
         ]
+      },
+      "max_lines": {
+        "description": "Wave 6 (item b, \"response budget for understand\"): same meaning as\n`SourceParams::max_lines` -- caps how many lines of the embedded\n`source.source` come back in one call. `None` (default) is today's\nunlimited behavior. When set and the resolved symbol has more lines\nthan this, `source.truncated`/`source.next_cursor` are populated\nthe same way `source()` itself reports them (pass `next_cursor`\nback as `resume_from_line` to continue).",
+        "type": [
+          "integer",
+          "null"
+        ],
+        "format": "int64"
+      },
+      "resume_from_line": {
+        "description": "Wave 6 (item b): same meaning as `SourceParams::resume_from_line`\n-- 1-indexed absolute line to resume reading the embedded source\nfrom, pairing with a prior response's `source.next_cursor`.",
+        "type": [
+          "integer",
+          "null"
+        ],
+        "format": "int64"
+      },
+      "max_chars": {
+        "description": "Wave 6 (item b/d): same meaning as `SourceParams::max_chars` --\nhard character cap on the embedded `source.source` text, applied\non top of whatever `max_lines` already selected.",
+        "type": [
+          "integer",
+          "null"
+        ],
+        "format": "int64"
       }
     }
   },