feat(invisible): vendor-adaptive default strength (OpenAI 0.10 / Google 0.15)

The default img2img strength is now chosen from the detected SynthID vendor
(C2PA issuer) instead of a single fixed 0.30: OpenAI gpt-image -> 0.10, Google
Gemini -> 0.15, unknown source -> 0.15. Explicit --strength always wins.

Basis: an oracle-verified June 2026 controlled study (clean v0.8.6, text/face
protection OFF, per-image openai.com/verify or Gemini-app verdict). OpenAI's
SynthID clears at 0.05 across 1024-1600 px (n=4, resolution-independent);
Google's is ~3x more robust and needs 0.15 on the capped-1536 path (n=4). The
dominant factor is the VENDOR, not resolution. The earlier single 0.30 default
and the "resolution dependence" lore came from contaminated tests run with the
protect-text bug ON (issue #14) -- re-running those same 1600x1600 images clean
removes SynthID at 0.05.

`vendor_for_strength(path)` reads metadata.synthid_source on the ORIGINAL input
and is threaded through cli (invisible/all/batch) -> invisible_engine ->
watermark_remover -> resolve_strength(strength, profile, vendor), so display and
execution use the same vendor (the engine sees a temp path whose C2PA the visible
pass already stripped, so detection must happen in the CLI on the pristine
source). Caveat: Google's 0.15 was validated only on --max-resolution 1536;
native 2816 Gemini was not locally measurable (OOM on Apple Silicon) and is
pending GPU validation on raiw.cc.

Docs: docs/synthid.md sections 2.2/4.4/5.2 corrected (the contaminated
resolution-dependence findings replaced with the clean oracle-verified table);
README and CLAUDE.md updated; CLI --strength help reflects the adaptive default.

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

src/remove_ai_watermarks/cli.py

@@ -18,7 +18,7 @@ from typing import TYPE_CHECKING, Any, Literal
 import click
 
 from remove_ai_watermarks import __version__, watermark_registry
-from remove_ai_watermarks.noai.watermark_profiles import resolve_strength
+from remove_ai_watermarks.noai.watermark_profiles import resolve_strength, vendor_for_strength
 
 if TYPE_CHECKING:
     from collections.abc import Generator
@@ -452,7 +452,8 @@ def cmd_erase(
     "--strength",
     type=float,
     default=None,
-    help="Denoising strength (0.0-1.0). Default: 0.30 (SDXL SynthID threshold); ctrlregen uses 1.0.",
+    help="Denoising strength (0.0-1.0). Default: vendor-adaptive (OpenAI 0.10 / Google 0.15 / "
+    "unknown 0.15, from the C2PA issuer); ctrlregen uses 1.0.",
 )
 @click.option("--steps", type=int, default=50, help="Number of denoising steps. Default: 50.")
 @click.option(
@@ -527,9 +528,12 @@ def cmd_invisible(
         progress_callback=progress_cb,
     )
 
+    # Detect the SynthID vendor from the ORIGINAL (before processing strips C2PA) so the
+    # displayed and executed strength agree on the vendor-adaptive default.
+    vendor = vendor_for_strength(source)
     console.print(f"  Input:    {source.name}")
     console.print(f"  Pipeline: {pipeline}")
-    console.print(f"  Strength: {resolve_strength(strength, pipeline)}  Steps: {steps}")
+    console.print(f"  Strength: {resolve_strength(strength, pipeline, vendor)}  Steps: {steps}")
 
     t0 = time.monotonic()
     result_path = engine.remove_watermark(
@@ -543,6 +547,7 @@ def cmd_invisible(
         protect_text=protect_text,
         protect_faces=protect_faces,
         max_resolution=max_resolution,
+        vendor=vendor,
     )
     elapsed = time.monotonic() - t0
 
@@ -689,7 +694,8 @@ def cmd_identify(ctx: click.Context, source: Path, no_visible: bool, as_json: bo
     "--strength",
     type=float,
     default=None,
-    help="Invisible watermark denoising strength. Default: 0.30 (SDXL); ctrlregen uses 1.0.",
+    help="Invisible watermark denoising strength. Default: vendor-adaptive "
+    "(OpenAI 0.10 / Google 0.15 / unknown 0.15); ctrlregen uses 1.0.",
 )
 @click.option("--steps", type=int, default=50, help="Number of denoising steps for invisible removal.")
 @click.option(
@@ -818,7 +824,11 @@ def cmd_all(
                 progress_callback=progress_cb,
             )
 
-            console.print(f"    Strength: {resolve_strength(strength, pipeline)}  Steps: {steps}")
+            # Detect the vendor from the pristine ORIGINAL (`source`); `tmp_path` has
+            # already lost its C2PA to the visible-removal pass, so reading it would
+            # always resolve to the unknown-vendor default.
+            vendor = vendor_for_strength(source)
+            console.print(f"    Strength: {resolve_strength(strength, pipeline, vendor)}  Steps: {steps}")
             inv_engine.remove_watermark(
                 image_path=tmp_path,
                 output_path=tmp_path,
@@ -829,6 +839,7 @@ def cmd_all(
                 protect_text=protect_text,
                 protect_faces=protect_faces,
                 max_resolution=max_resolution,
+                vendor=vendor,
             )
             console.print("    Invisible watermark removed")
 
@@ -941,6 +952,9 @@ def _process_batch_image(
                 seed=seed,
                 humanize=humanize,
                 max_resolution=max_resolution,
+                # Detect the vendor from the pristine original (`img_path`), not the
+                # visible-processed `out_path` whose C2PA is already gone.
+                vendor=vendor_for_strength(img_path),
             )
 
     if mode in ("metadata", "all"):