feat: map Adobe STAMP field to DocuSign stampTabs

Adobe STAMP (hanko/seal) has a direct DocuSign equivalent via
stampTabs. Previously marked as skipped with no equivalent.

- compose_docusign_template.py: emit stampTabs for STAMP input type;
  PARTICIPATION_STAMP remains skipped (still no equivalent)
- field-mapping.md: update STAMP row, add stampTabs to multi-location
  non-merging list, add account feature prerequisite to Known Gaps

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

field-mapping.md

@@ -23,7 +23,7 @@ Source: Adobe Sign UI "Change field type" dropdown (all 15 types) + API field da
 | Image               | INLINE_IMAGE      | DATA             | —          | (skipped)            | No DocuSign equivalent                     |
 | Company             | TEXT_FIELD        | COMPANY or SIGNER_COMPANY | —   | companyTabs          | Auto-populated from signer profile. API returns `SIGNER_COMPANY` when set via UI. |
 | Title               | TEXT_FIELD        | TITLE or SIGNER_TITLE | —       | titleTabs            | Auto-populated from signer profile. API returns `SIGNER_TITLE` when set via UI. |
-| Stamp               | STAMP             | —                | —          | (skipped)            | No DocuSign equivalent                     |
+| Stamp               | STAMP             | —                | —          | stampTabs            | DocuSign stampTabs — signer uploads/selects a stamp image (hanko/seal). Requires stamp feature enabled on account. |
 | Signature block     | BLOCK             | SIGNATURE_BLOCK  | —          | signHereTabs         | Composite block — mapped to sign-here      |
 
 ## Role/Recipient Mapping
@@ -76,7 +76,7 @@ Tab types that support merging (one tab emitted per location):
 `emailAddressTabs`, `companyTabs`, `titleTabs`, `listTabs`, `checkboxTabs`
 
 Tab types that do not merge (only first location used or handled specially):
-`signHereTabs`, `initialHereTabs` — each location is an independent signing action
+`signHereTabs`, `initialHereTabs`, `stampTabs` — each location is an independent signing/stamping action
 `radioGroupTabs` — each location is one radio button within the group
 `signerAttachmentTabs` — each location is an independent attachment request
 
@@ -90,6 +90,9 @@ Tab types that do not merge (only first location used or handled specially):
   best-effort via standard DocuSign validation types only.
 - **Radio group flattening**: Adobe radios with `radioGroup` are merged into a single
   DocuSign `radioGroupTabs` entry with per-location radio button coordinates.
+- **Stamp tab account feature**: `stampTabs` requires the stamp/hanko feature to be
+  enabled on the DocuSign account. Verify before migrating templates that contain
+  Adobe Sign STAMP fields.
 
 ## To Do
 - Add conditional logic/rule mapping table

src/compose_docusign_template.py

@@ -19,9 +19,10 @@ Key rules applied:
   - No top-level "status" field (belongs on envelope sends, not templates)
 
 Field type coverage:
-  Mapped:   TEXT_FIELD, SIGNATURE, CHECKBOX, DATE, DROP_DOWN, RADIO, BLOCK
+  Mapped:   TEXT_FIELD, SIGNATURE, CHECKBOX, DATE, DROP_DOWN, RADIO, BLOCK, STAMP
   Partial:  FILE_CHOOSER → signerAttachmentTabs (with warning)
-  Skipped:  INLINE_IMAGE (no DocuSign equivalent — warning logged)
+            STAMP → stampTabs (requires stamp feature enabled on DocuSign account — warning logged)
+  Skipped:  INLINE_IMAGE, PARTICIPATION_STAMP (no DocuSign equivalent — warning logged)
 """
 
 import base64
@@ -229,8 +230,14 @@ def build_tabs_for_field(field: dict, warnings: list) -> dict:
         warnings.append(f"INLINE_IMAGE '{label}' → skipped (no DocuSign equivalent)")
         return {}
 
-    elif input_type in ("STAMP", "PARTICIPATION_STAMP"):
-        warnings.append(f"{input_type} '{label}' → skipped (no DocuSign equivalent)")
+    elif input_type == "STAMP":
+        # DocuSign stampTabs — signer uploads or selects a hanko/seal stamp image.
+        # Requires the stamp feature to be enabled on the DocuSign account.
+        warnings.append(f"STAMP '{label}' → stampTabs (verify stamp feature is enabled on your DocuSign account)")
+        return {"stampTabs": [_make_base_tab(loc, label) for loc in locations]}
+
+    elif input_type == "PARTICIPATION_STAMP":
+        warnings.append(f"PARTICIPATION_STAMP '{label}' → skipped (no DocuSign equivalent)")
         return {}
 
     else: