[NOTICKET] feat(knowledge-parsing): add 1-based page_no (S6b)

A computed `page_no` (= page_idx + 1) on Chunk, so the human/review-UI page
number lives in the artifact instead of each consumer converting it.

- Derived, never stored: cannot drift from page_idx, and neither normaliser has
to set it. page_idx stays 0-based and raw.
- Additive, schema 0.3.2 -> 0.3.3; older artifacts and consumers are unaffected.
- Closes S6b (open since 2026-08-19); reverses the earlier "convert in the UI"
note now that a real reviewer is about to read these.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/knowledge_parsing/contracts.py

@@ -39,15 +39,16 @@ from __future__ import annotations
 
 from typing import Literal
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, computed_field
 
 ChunkKind = Literal["text", "table", "chart", "equation", "image"]
 
+# 0.3.3 — adds the computed 1-based `page_no` (S6b). Derived from `page_idx`.
 # 0.3.2 — adds the optional `source_title`.
 # 0.3.1 — adds the optional `normaliser_version`.
-# Both patch bumps: purely additive, so every 0.3.0 consumer reads them unchanged
+# All patch bumps: purely additive, so every 0.3.0 consumer reads them unchanged
 # and the new fields are null on anything written earlier.
-SCHEMA_VERSION = "0.3.2"
+SCHEMA_VERSION = "0.3.3"
 
 
 class Chunk(BaseModel):
@@ -70,15 +71,26 @@ class Chunk(BaseModel):
 
     # Location in the source document.
     #
-    # Named `page_idx` deliberately: these are 0-BASED, exactly as MinerU reports
-    # them, with no conversion anywhere in the pipeline. Page numbers eventually
-    # reach a human review queue, and an off-by-one there is invisible until an
-    # expert opens the wrong page. Converting to 1-based is the UI's job, done
-    # once at display time — never here, so the artifact always matches the raw
-    # MinerU output kept alongside it.
+    # Named `page_idx` deliberately: these are 0-BASED, exactly as the parser
+    # reports them, with no conversion — so the artifact always matches the raw
+    # parser output kept alongside it. The 1-based number a human reads is
+    # `page_no` below: a review queue is about to see these, and an off-by-one
+    # there is invisible until an expert opens the wrong page, so the conversion
+    # is done once, here, rather than left to each consumer (S6b, 2026-09-02).
     page_idx: int
     page_idxs: list[int] = Field(default_factory=list)
 
+    @computed_field  # type: ignore[prop-decorator]
+    @property
+    def page_no(self) -> int:
+        """1-based page number, for humans and the review UI.
+
+        Derived from `page_idx`, never stored, so it cannot drift from it — and
+        neither normaliser has to remember to set it. `page_idx` stays 0-based
+        and raw; this is the single number the reviewer sees.
+        """
+        return self.page_idx + 1
+
     # Structural context. All Optional — many documents carry no headings.
     section_no: str | None = None  # e.g. "2.1.3", when the document is numbered