fix: EC rebalance fails with replica placement 000 (#7812)

* fix: EC rebalance fails with replica placement 000

This PR fixes several issues with EC shard distribution:

1. Pre-flight check before EC encoding
   - Verify target disk type has capacity before encoding starts
   - Prevents encoding shards only to fail during rebalance
   - Shows helpful error when wrong diskType is specified (e.g., ssd when volumes are on hdd)

2. Fix EC rebalance with replica placement 000
   - When DiffRackCount=0, shards should be distributed freely across racks
   - The '000' placement means 'no volume replication needed' because EC provides redundancy
   - Previously all racks were skipped with error 'shards X > replica placement limit (0)'

3. Add unit tests for EC rebalance slot calculation
   - TestECRebalanceWithLimitedSlots: documents the limited slots scenario
   - TestECRebalanceZeroFreeSlots: reproduces the 0 free slots error

4. Add Makefile for manual EC testing
   - make setup: start cluster and populate data
   - make shell: open weed shell for EC commands
   - make clean: stop cluster and cleanup

* fix: default -rebalance to true for ec.encode

The -rebalance flag was defaulting to false, which meant ec.encode would
only print shard moves but not actually execute them. This is a poor
default since the whole point of EC encoding is to distribute shards
across servers for fault tolerance.

Now -rebalance defaults to true, so shards are actually distributed
after encoding. Users can use -rebalance=false if they only want to
see what would happen without making changes.

* test/erasure_coding: improve Makefile safety and docs

- Narrow pkill pattern for volume servers to use TEST_DIR instead of
  port pattern, avoiding accidental kills of unrelated SeaweedFS processes
- Document external dependencies (curl, jq) in header comments

* shell: refactor buildRackWithEcShards to reuse buildEcShards

Extract common shard bit construction logic to avoid duplication
between buildEcShards and buildRackWithEcShards helper functions.

* shell: update test for EC replication 000 behavior

When DiffRackCount=0 (replication "000"), EC shards should be
distributed freely across racks since erasure coding provides its
own redundancy. Update test expectation to reflect this behavior.

* erasure_coding: add distribution package for proportional EC shard placement

Add a new reusable package for EC shard distribution that:
- Supports configurable EC ratios (not hard-coded 10+4)
- Distributes shards proportionally based on replication policy
- Provides fault tolerance analysis
- Prefers moving parity shards to keep data shards spread out

Key components:
- ECConfig: Configurable data/parity shard counts
- ReplicationConfig: Parsed XYZ replication policy
- ECDistribution: Target shard counts per DC/rack/node
- Rebalancer: Plans shard moves with parity-first strategy

This enables seaweed-enterprise custom EC ratios and weed worker
integration while maintaining a clean, testable architecture.

* shell: integrate distribution package for EC rebalancing

Add shell wrappers around the distribution package:
- ProportionalECRebalancer: Plans moves using distribution.Rebalancer
- NewProportionalECRebalancerWithConfig: Supports custom EC configs
- GetDistributionSummary/GetFaultToleranceAnalysis: Helper functions

The shell layer converts between EcNode types and the generic
TopologyNode types used by the distribution package.

* test setup

* ec: improve data and parity shard distribution across racks

- Add shardsByTypePerRack helper to track data vs parity shards
- Rewrite doBalanceEcShardsAcrossRacks for two-pass balancing:
  1. Balance data shards (0-9) evenly, max ceil(10/6)=2 per rack
  2. Balance parity shards (10-13) evenly, max ceil(4/6)=1 per rack
- Add balanceShardTypeAcrossRacks for generic shard type balancing
- Add pickRackForShardType to select destination with room for type
- Add unit tests for even data/parity distribution verification

This ensures even read load during normal operation by spreading
both data and parity shards across all available racks.

* ec: make data/parity shard counts configurable in ecBalancer

- Add dataShardCount and parityShardCount fields to ecBalancer struct
- Add getDataShardCount() and getParityShardCount() methods with defaults
- Replace direct constant usage with configurable methods
- Fix unused variable warning for parityPerRack

This allows seaweed-enterprise to use custom EC ratios while
defaulting to standard 10+4 scheme.

* Address PR 7812 review comments

Makefile improvements:
- Save PIDs for each volume server for precise termination
- Use PID-based killing in stop target with pkill fallback
- Use more specific pkill patterns with TEST_DIR paths

Documentation:
- Document jq dependency in README.md

Rebalancer fix:
- Fix duplicate shard count updates in applyMovesToAnalysis
- All planners (DC/rack/node) update counts inline during planning
- Remove duplicate updates from applyMovesToAnalysis to avoid double-counting

* test/erasure_coding: use mktemp for test file template

Use mktemp instead of hardcoded /tmp/testfile_template.bin path
to provide better isolation for concurrent test runs.

.gitignore

@@ -130,3 +130,4 @@ coverage.out
 /test/s3/remote_cache/test-remote-data
 test/s3/remote_cache/remote-server.pid
 test/s3/remote_cache/primary-server.pid
+/test/erasure_coding/filerldb2