docs: define native print capture architecture

PRINT_CAPTURE_ARCHITECTURE.md

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+# FieldLens Print Capture Architecture
+
+## Target architecture
+
+### Linux
+
+```text
+original document
+  -> FieldLens Print CUPS queue
+  -> FieldLens CUPS pre-filter
+       -> /var/lib/fieldlens-print/capture
+       -> FieldLens watcher / decode / sync
+  -> normal CUPS rendering
+  -> physical printer
+```
+
+The production Linux path does not depend on the RAW 9100 proxy. The CUPS pre-filter is authoritative for original-document capture.
+
+The no-paper self-test uses the same CUPS pre-filter, then sends the rendered output to a local discard sink on 127.0.0.1:9101. Self-test success means the exact submitted document was observed by the FieldLens watcher, not merely accepted by CUPS.
+
+### Windows
+
+Target:
+
+```text
+original spool/document
+  -> FieldLens native capture backend
+       -> FieldLens capture / decode / sync
+  -> normal Windows print pipeline
+  -> physical printer
+```
+
+The current localhost:9100 RAW proxy remains a compatibility backend only until the native Windows backend is complete.
+
+## Windows native backend implementation track
+
+1. Spike the supported Windows spool interception options:
+   - custom port monitor,
+   - print processor,
+   - virtual printer / print support backend.
+2. Prefer the least-privileged option that can capture the spool/document before final device rendering while returning the job to the normal Windows print pipeline.
+3. Package and register the native component from the FieldLens installer with repair/remove support.
+4. Preserve the existing FieldLens capture event contract so OCR/barcode, history, sync, and UI do not care which platform backend produced the job.
+5. Support USB, WSD, locally installed, and network printers without requiring destination TCP/9100.
+6. Add health checks that verify the native backend is installed, loadable, and attached to the FieldLens queue.
+7. Add an end-to-end self-test that proves a submitted document creates a FieldLens capture event and reaches the downstream Windows print pipeline.
+8. Remove the localhost:9100 compatibility backend after native backend rollout and migration are verified.
+
+## Health invariants
+
+FieldLens must not report Ready unless the active platform capture backend is actually operational.
+
+Linux Ready requires:
+- FieldLens Print queue present, enabled, and accepting jobs;
+- FieldLens Test Capture queue present, enabled, and accepting jobs;
+- installed FieldLens CUPS filter present and Python-syntax-valid;
+- production capture watcher running.
+
+Linux self-test passes only when the exact submitted test document creates a matching `cups-test` capture event.
+
+Windows Ready will eventually require the equivalent native-backend registration/load/queue checks, rather than only queue existence.