flow/designs: add per-PDK WNS plots and README from rules-base.json

Add flow/util/plot_wns.py, a local (no-bazel) script that reads the
committed rules-base.json baselines and regenerates, per PDK, a
worst-setup-slack bar chart (wns.png) and a "## WNS" README section
between generated markers. It covers all 9 PDKs that ship timing
baselines (76 designs); the bar is finish-stage WNS, with cts and
globalroute drawn as markers so stage-to-stage movement is visible.

No OpenROAD/ORFS run is required -- the data already lives in the tree,
so the plots are deterministic and the committed PNGs + tables render
on GitHub with nothing to run. asap7/README.md additionally carries a
hand-written findings section discussing the results.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

Author: oharboe

flow/designs/asap7/README.md

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+# asap7 designs
+
+## Findings: worst negative slack (WNS) across asap7
+
+These notes read the committed `rules-base.json` baselines — the worst setup slack each
+design reaches at clock-tree synthesis (`cts`), global route (`globalroute`) and `finish`.
+The plot and table below are regenerated from that data by
+[`flow/util/plot_wns.py`](../../util/plot_wns.py); no flow run is needed, so the numbers
+are exactly what CI checks against.
+
+What stands out:
+
+- **Every asap7 design closes with negative setup slack.** All 18 baselines have
+  `finish` WNS < 0, so the committed asap7 constraints target clocks the flow does not
+  actually meet — these baselines track *how far short* each design lands, not timing
+  closure. `swerv_wrapper` (≈ −318) and `mock-alu` (≈ −300) are the extreme cases, an
+  order of magnitude worse than the cluster around −15 … −50 (`aes*`, `mock-cpu`,
+  `uart`, `jpeg`).
+
+- **Worst slack is often not stable across stages.** Some designs pin their critical
+  path early and barely move (`cva6`, `jpeg`, `mock-cpu`: `cts ≈ globalroute ≈ finish`).
+  Others move a lot: `swerv_wrapper` degrades from −80 at cts/globalroute to −318 at
+  finish, and `riscv32i`/`riscv32i-mock-sram` slip from ≈ −47 (cts) to ≈ −81 (finish) —
+  routing and final extraction make the path materially worse than CTS predicted.
+
+- **Global route is sometimes more pessimistic than finish.** For `gcd`
+  (cts −85.9 → globalroute −110 → finish −96.7) and `aes-block`
+  (cts −84 → globalroute −31 → finish −57) the worst-slack estimate swings between
+  stages rather than monotonically worsening. This is the same GRT-vs-post-route
+  estimate gap explored in [`flow/docs/rcx`](../../docs/rcx/README.md) (PR #4302), here
+  visible at the design level rather than per net.
+
+The takeaway for anyone using cts-stage slack as a proxy for the final result: for asap7
+it is a usable rank ordering but not a reliable magnitude — several designs move tens of
+units (and `swerv_wrapper` hundreds) between cts and finish.
+
+<!-- BEGIN WNS (generated by flow/util/plot_wns.py) -->
+## WNS
+
+Worst setup slack per design at three flow stages — clock-tree synthesis (`cts`), global route (`globalroute`) and `finish` — read from each design's `rules-base.json`. Negative means setup timing is not met. Values are in this PDK's native timing unit (ps for `asap7`, ns for most others), so they are comparable within this PDK but not across PDKs.
+
+The bar is the `finish` slack; the markers show the `cts` and `globalroute` slack for the same design, so stage-to-stage movement is visible.
+
+![WNS by design — asap7](wns.png)
+
+| design | cts | globalroute | finish |
+| --- | ---: | ---: | ---: |
+| swerv_wrapper | -80 | -80 | -318 |
+| mock-alu | -289 | -303 | -300 |
+| gcd | -85.9 | -110 | -96.7 |
+| gcd-ccs | -101 | -93.7 | -96.4 |
+| ethmac | -88.6 | -103 | -93.3 |
+| riscv32i-mock-sram | -47.5 | -52.8 | -81.3 |
+| riscv32i | -47.5 | -49.8 | -81.2 |
+| jpeg_lvt | -30 | -30 | -63.9 |
+| aes-block | -84.1 | -31.1 | -56.6 |
+| ibex | -79.4 | -74.3 | -52.5 |
+| cva6 | -50 | -50 | -50 |
+| uart | -47.6 | -58.7 | -49.1 |
+| jpeg | -34 | -34 | -34 |
+| aes_lvt | -18 | -18 | -26.1 |
+| aes | -28.9 | -28 | -24.2 |
+| aes-mbff | -20.8 | -19 | -20.8 |
+| mock-cpu | -16.6 | -16.6 | -16.6 |
+| ethmac_lvt | -19 | -29.5 | -15.2 |
+
+_Generated by `flow/util/plot_wns.py` from `rules-base.json`; regenerate with `python3 flow/util/plot_wns.py`._
+<!-- END WNS -->

flow/designs/asap7/wns.png

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+# gf12 designs
+
+<!-- BEGIN WNS (generated by flow/util/plot_wns.py) -->
+## WNS
+
+Worst setup slack per design at three flow stages — clock-tree synthesis (`cts`), global route (`globalroute`) and `finish` — read from each design's `rules-base.json`. Negative means setup timing is not met. Values are in this PDK's native timing unit (ps for `asap7`, ns for most others), so they are comparable within this PDK but not across PDKs.
+
+The bar is the `finish` slack; the markers show the `cts` and `globalroute` slack for the same design, so stage-to-stage movement is visible.
+
+![WNS by design — gf12](wns.png)
+
+| design | cts | globalroute | finish |
+| --- | ---: | ---: | ---: |
+| bp_single | -632 | -470 | -227 |
+| bp_quad | -375 | -223 | -225 |
+| ariane | -257 | -214 | -212 |
+| coyote | -200 | -200 | -200 |
+| bp_dual | -758 | -100 | -165 |
+| ca53 | -100 | -100 | -100 |
+| swerv_wrapper | -75 | -75 | -75 |
+| ibex | -51 | -51 | -51 |
+| tinyRocket | -40 | -40 | -40 |
+| jpeg | -96 | -53.2 | -25 |
+| aes | -21 | -21 | -21 |
+| gcd | -21.5 | -26.3 | -14 |
+| ariane133 | -20.2409 | 0 | 0 |
+
+_Generated by `flow/util/plot_wns.py` from `rules-base.json`; regenerate with `python3 flow/util/plot_wns.py`._
+<!-- END WNS -->

flow/designs/gf12/README.md

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+# gf180 designs
+
+<!-- BEGIN WNS (generated by flow/util/plot_wns.py) -->
+## WNS
+
+Worst setup slack per design at three flow stages — clock-tree synthesis (`cts`), global route (`globalroute`) and `finish` — read from each design's `rules-base.json`. Negative means setup timing is not met. Values are in this PDK's native timing unit (ps for `asap7`, ns for most others), so they are comparable within this PDK but not across PDKs.
+
+The bar is the `finish` slack; the markers show the `cts` and `globalroute` slack for the same design, so stage-to-stage movement is visible.
+
+![WNS by design — gf180](wns.png)
+
+| design | cts | globalroute | finish |
+| --- | ---: | ---: | ---: |
+| aes-hybrid | -0.967 | -1.1 | -1.1 |
+| aes | -0.789 | -0.876 | -0.918 |
+| riscv32i | -0.586 | -0.659 | -0.63 |
+| ibex | -0.5 | -0.5 | -0.539 |
+| jpeg | -0.375 | -0.375 | -0.375 |
+| uart-blocks | -0.3 | -0.3 | -0.3 |
+
+_Generated by `flow/util/plot_wns.py` from `rules-base.json`; regenerate with `python3 flow/util/plot_wns.py`._
+<!-- END WNS -->

flow/designs/gf12/wns.png

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+# gf55 designs
+
+<!-- BEGIN WNS (generated by flow/util/plot_wns.py) -->
+## WNS
+
+Worst setup slack per design at three flow stages — clock-tree synthesis (`cts`), global route (`globalroute`) and `finish` — read from each design's `rules-base.json`. Negative means setup timing is not met. Values are in this PDK's native timing unit (ps for `asap7`, ns for most others), so they are comparable within this PDK but not across PDKs.
+
+The bar is the `finish` slack; the markers show the `cts` and `globalroute` slack for the same design, so stage-to-stage movement is visible.
+
+![WNS by design — gf55](wns.png)
+
+| design | cts | globalroute | finish |
+| --- | ---: | ---: | ---: |
+| aes |  |  | 0 |
+
+_Generated by `flow/util/plot_wns.py` from `rules-base.json`; regenerate with `python3 flow/util/plot_wns.py`._
+<!-- END WNS -->

flow/designs/gf180/README.md

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+# gt2n designs
+
+<!-- BEGIN WNS (generated by flow/util/plot_wns.py) -->
+## WNS
+
+Worst setup slack per design at three flow stages — clock-tree synthesis (`cts`), global route (`globalroute`) and `finish` — read from each design's `rules-base.json`. Negative means setup timing is not met. Values are in this PDK's native timing unit (ps for `asap7`, ns for most others), so they are comparable within this PDK but not across PDKs.
+
+The bar is the `finish` slack; the markers show the `cts` and `globalroute` slack for the same design, so stage-to-stage movement is visible.
+
+![WNS by design — gt2n](wns.png)
+
+| design | cts | globalroute | finish |
+| --- | ---: | ---: | ---: |
+| aes | -25 | -75.2 | -75.2 |
+| jpeg | -50 | -50 | -50 |
+| gcd | -25 | -25 | -25 |
+
+_Generated by `flow/util/plot_wns.py` from `rules-base.json`; regenerate with `python3 flow/util/plot_wns.py`._
+<!-- END WNS -->