feat(hw): add v2.9 placeholders

Add placeholder directory structure for PCB v2.9 hardware revision:
- hardware/v2.9/ with altium, bom, easyeda, manufacturing, parsed_data
- README placeholders describing expected contents and open decisions
- v2.9 placeholder entry in savearth_hw_knowledge.json
  (component_count: 0, empty components, evolution diff from v2.8)

Links to docs/v2.9_mitigation_plan.md for architecture details.

hardware/v2.9/README.md

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+# Savearth PCB v2.9 — Placeholder
+
+This directory is a placeholder for the upcoming **PCB v2.9** hardware revision.
+
+## Purpose
+
+PCB v2.9 addresses the architectural flaws identified in v2.7 and v2.8:
+
+| Issue | v2.7 | v2.8 | v2.9 Target |
+|-------|------|------|-------------|
+| Reed switch pin | `GPIO43` (TX0) | PCAL9538A P5 | Dedicated RTC GPIO |
+| Deep-sleep wake | Yes (with UART conflict) | No | Yes via `EXT1` |
+| Hard reset | Cut regulator power | None (always-on) | Supervisor IC / delay circuit |
+| Factory flashing | Blocked by reed switch | Tweezers on CON header | On-board BOOT + RESET tactile buttons |
+
+## Required Architectural Changes
+
+1. **Reed Switch → RTC GPIO**
+   - Move the reed switch from the PCAL9538A I/O expander to a dedicated, non-strapping RTC GPIO on the ESP32-S3.
+   - Candidate pins: `GPIO4`, `GPIO5`, or another RTC-capable pin that does not conflict with boot strapping (`GPIO0`, `GPIO3`, `GPIO45`, `GPIO46`).
+
+2. **Hardware Hard-Reset Circuit**
+   - Long magnet hold (> 10 s) must unconditionally pull `CHIP_PU` (`EN`) LOW.
+   - Options under evaluation:
+     - Dedicated supervisor IC (e.g., LTC2950, MAX16054, TPS3420).
+     - Passive RC delay + MOSFET / comparator.
+
+3. **Soft-Latch Power Circuit (optional / TBD)**
+   - Replace the always-on `EN`-to-`VSYS` connection with a soft-latch that the firmware can release for true zero-power shutdown.
+   - Requires a `POWER_HOLD` GPIO available immediately at boot.
+
+4. **Tactile Buttons**
+   - Add `RESET` button: `CHIP_PU` → GND (100 nF debounce).
+   - Add `BOOT` button: `GPIO0` → GND (100 nF debounce).
+
+## Directory Layout
+
+```
+hardware/v2.9/
+├── altium/                 # Altium Designer project placeholders
+├── bom/                    # Bill of Materials placeholders
+├── easyeda/                # EasyEDA Professional project placeholders
+├── manufacturing/          # Gerber, pick-and-place, 3D STEP placeholders
+└── parsed_data/            # Extracted schematic/BOM/netlist placeholders
+```
+
+## Open Decisions
+
+- [ ] RTC GPIO selection for reed switch.
+- [ ] Supervisor IC vs. passive RC for hard reset.
+- [ ] `POWER_HOLD` GPIO selection (if soft-latch is implemented).
+- [ ] Boot/reset button footprint and enclosure cutout strategy.
+- [ ] BOM cost target and budget for added components.
+- [ ] PIR noise: hardware RC filter required or firmware-only debounce?
+
+## Related Documents
+
+- [`docs/v2.9_mitigation_plan.md`](/docs/v2.9_mitigation_plan.md)
+- [`docs/hw_meeting_v28_v29_agenda.md`](/docs/hw_meeting_v28_v29_agenda.md)
+- [`docs/v2.8_hardware_analysis_and_fixes.md`](/docs/v2.8_hardware_analysis_and_fixes.md)
+- Jira: HW-52, HW-55, HW-56, HW-57, HW-58

hardware/v2.9/altium/README.md

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+# Altium Project Placeholder — PCB v2.9
+
+This directory will hold the Altium Designer project files for PCB v2.9.
+
+Expected files:
+
+- `Savearth_v2_9.PrjPcb` — Altium project file.
+- `Savearth_v2_9.pcbdoc` — PCB layout.
+- `Savearth_v2_9/1_Savearth SCH ESP32 v2_9.schdoc` — ESP32/MCU schematic.
+- `Savearth_v2_9/2_Savearth SCH Power v2_9.schdoc` — Power management schematic.

hardware/v2.9/bom/README.md

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+# BOM Placeholder — PCB v2.9
+
+This directory will contain the v2.9 bill of materials once the schematic is finalized.
+
+Expected files:
+
+- `bom_v2_9.json` — Structured BOM extracted from the CAD tool.
+- `bom_v2_9_jlcpcb.csv` — JLCPCB-compatible BOM for assembly quoting.
+
+## TBD Components
+
+The v2.9 BOM is expected to add one or more of the following, depending on the architecture decisions:
+
+- Reset / push-button supervisor IC (or RC delay components).
+- P-MOSFET + N-MOSFET + passives for soft-latch power (if implemented).
+- Two SMD tactile switches (BOOT, RESET) with debounce capacitors.
+- Optional PIR output RC filter components.
+
+Until the schematic is ready, this directory intentionally contains only this placeholder.

hardware/v2.9/easyeda/README.md

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+# EasyEDA Project Placeholder — PCB v2.9
+
+This directory will hold the EasyEDA Professional project files for PCB v2.9.
+
+Expected files:
+
+- `Savearth_v2_9.epro` — Top-level EasyEDA project.
+- `Savearth SCH ESP32 v2_9.epro` — ESP32/MCU subsystem schematic.
+- `Savearth SCH Power v2_9.epro` — Power management subsystem schematic.
+- `Savearth GBT.eprj` — Optional local library/project archive.
+
+The v2.9 schematic will implement the mitigations documented in `docs/v2.9_mitigation_plan.md`.

hardware/v2.9/manufacturing/README.md

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+# Manufacturing Outputs Placeholder — PCB v2.9
+
+This directory will contain the manufacturing deliverables for PCB v2.9 once the layout is complete.
+
+Expected files:
+
+- `Gerber_Savearth_v2_9_YYYY-MM-DD.zip` — Gerber RS-274X outputs.
+- `PickAndPlace_Savearth_v2_9_YYYY-MM-DD.xlsx` — Centroid / pick-and-place file.
+- `BOM_Savearth_v2_9_YYYY-MM-DD.xlsx` — Manufacturing BOM.
+- `3D_Savearth_v2_9_YYYY-MM-DD.step` — 3D mechanical model.

hardware/v2.9/parsed_data/README.md

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+# Parsed Data Placeholder — PCB v2.9
+
+This directory will contain extracted/scraped data from the v2.9 CAD sources for automated analysis.
+
+Expected files:
+
+- `schematic_raw.json` — Raw component/nets extracted from the schematic.
+- `bom_extended.json` — Enriched BOM with supplier data.
+- `netlist.json` — Connectivity netlist for automated checks.