feat: unify object observation API

crates/wip-fs-server/README.md

@@ -68,29 +68,29 @@ The following examples use development HTTP for readability. The JSON is the
 standard `wip-https` profile; validators and operation `Bytes` are base64 body
 fields, never `ETag` or `If-Match` headers.
 
-Fetch root:
+Observe only the root Object:
 
 ```console
-curl -sS -X POST http://127.0.0.1:8080/wip/v1/fetch \
+curl -sS -X POST http://127.0.0.1:8080/wip/v1/observe \
   -H 'Content-Type: application/json' -H 'Accept: application/json' \
-  --data '{"path":"/"}'
+  --data '{"path":"/","depth":0}'
 ```
 
-Fetch a bounded tree (the root counts toward `limit`):
+Observe the root and two levels of indexable children:
 
 ```console
-curl -sS -X POST http://127.0.0.1:8080/wip/v1/fetch_tree \
+curl -sS -X POST http://127.0.0.1:8080/wip/v1/observe \
   -H 'Content-Type: application/json' -H 'Accept: application/json' \
-  --data '{"path":"/","depth":2,"limit":100}'
+  --data '{"path":"/","depth":2}'
 ```
 
-A regular-file fetch returns the opaque read interface reference and an object
+A regular-file observation returns the opaque read interface reference and an object
 validator. Retrieve that descriptor with:
 
 ```console
 curl -sS -X POST http://127.0.0.1:8080/wip/v1/fetch_interface \
   -H 'Content-Type: application/json' -H 'Accept: application/json' \
-  --data '{"interface":"<read-interface-from-fetch>"}'
+  --data '{"interface":"<read-interface-from-observation>"}'
 ```
 
 Then call a bounded range read:
@@ -120,8 +120,8 @@ curl -sS -X POST http://127.0.0.1:8080/wip/v1/call_operation \
   }'
 ```
 
-`fetch`, `fetch_tree`, and `fetch_interface` can also be combined through the
-standard `POST /wip/v1/batch` route. `call_operation` is never batchable.
+`observe` and `fetch_interface` can also be combined through the standard
+`POST /wip/v1/batch` route. `call_operation` is never batchable.
 
 ## Resource and failure behavior
 
@@ -133,12 +133,11 @@ stalled clients are closed and cannot occupy a worker beyond that bound. Workers
 accept connections directly, so the listener never moves more than the configured
 concurrency into active userspace service. Advisory file-lock acquisition uses a
 nonblocking loop and checks the same deadline/cancellation rather than allowing a
-contended lock to stall a worker. Tree traversal is breadth-first and opens only
-requested entries. Once a limit or depth boundary is reached it reads only far
-enough to
-prove whether an additional visible child exists, so `truncated` is true only
-when something was actually omitted. I/O failure and cancellation remain errors,
-not clean exhaustion.
+contended lock to stall a worker. Object observation recursively opens only the
+requested depth. A successful response is complete through that depth; if the
+configured projected-node budget would be exceeded, the interaction fails with
+`ResourceLimitExceeded` instead of returning a partial observation. I/O failure
+and cancellation remain errors.
 
 Whole-file replacement compares the request validator against the identity and
 state of the same pinned file handle used for the write. Failures before mutation