3f-fluid fluid lambdas (valid/retry/fire)

Transactional blocks where each input/output carries a valid/retry handshake; a transaction advances only when .[fire] (.[valid] and !.[retry]). Callable only from mod/fluid. The frontend parses the fluid kind (incl. pub fluid), but nothing lowers the elastic machinery and no test exercises it.

Failing example (pyrope/verilog pair, sketch)

// finc.prp — combinational fluid stage: handshake passes through
fluid finc(a:u8) -> (r:u9) {
  r = a + 1
}

// finc.v — expected mapping (port naming per the fluid ABI to be fixed)
module finc(input  [7:0] a, input  a_valid, output a_retry,
            output [8:0] r, output r_valid, input  r_retry);
  assign r       = a + 1;
  assign r_valid = a_valid;
  assign a_retry = r_retry;
endmodule

Docs (TBD markers)

• docs/pyrope/06d-fluid.md — chapter TBD admonition
• docs/pyrope/06-functions.md — lambda-kinds list, fluid bullet
• docs/pyrope/15-tbd.md

Done when

The pair LECs (after fixing the handshake ABI), .[fire] gating and registered fluid stages work, and 06d's examples elaborate. Then drop the TBD marks in 06d/06/quick-intro.